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CONVERSATIONS 

ON 
IN WHICH 

THE ELEMENTS OF THAT SCIENCE 

ARE 

FAMILIARLY EXPLAINED, 

AND ADAPTED TO THE 

COMPREHENSION OF YOUNG PUPILS. 

BY THE AUTHOR OF CONVERSATIONS ON CHEMISTRY, AND 
CONVERSATIONS ON POLITICAL ECONOMY. 



WITH 

ADDITIONAL ILLUSTRATIONS 

AND 

APPROPRIATE QUESTIONS 

FOR THE 

EXAMINATION OF SCHOLARS, 

BT 

REV. J. L. BLAKE, A. M. 

Principal of the Literary Seminary, Concord, New Hampshire. 



FlfTB AMERICAJr KDITIOJr. 



HARTFORD : 

PRIKTBD BT GEORGB «OODWIN. 

1823. 



DISTRICT OF NEW-HAMPSHIRE :-TO WIT. 

District Clerk's Office, 

»********« T) E it remembei-ed, that on the twenty-seventh day of March, A. D, 

» L S * JlJ 18-23, and in the forty-seventh year of the Independence of the 

* • * J Umted States of America, JOHN LAURIS BLAKE, of the said District, 

»«««*««*«* ^3th deposited in this office the title of a book, the right whereof he 

claims as author in the words following, viz. " Conversations on Natural Philosophy, 

*' in which the elements of that science are familiarly explained, and adapted to the 

*' comprehension of young pupils. Illustrated with plates, ay the author of Conver- 

** satious on Chemistry, and Conversations on Political Economy. With additional 

*' ilhisirationsand appropriate questions for the examination of scholars, by Rev. J. L. 

•* BLAKE, A. >L Principal of the Literary Seminai7, Concord, New Hampshire." 

In conformity to the act of the Congress of the United States, entitled " An act for 
the encouragement of learning by securing the copies of maps, charts and books to the 
authors and proprietors of such copies, during the times therein mentioned;" and also 
to an act ir-ntitkd '• an act supplementary to an act entitled an act for the encourage- 
ment of 1 .arning, by securing the copies of maps, charts and books to the authors and 
proprietorsof such copies, durinj? the times therein mentioned, and extending the ben- 
efits thereof to the arts of designing, engraving and etching historical and other prints." 
WILLIAM CLAGGETT, Clerk of the 

District of New- Hampshire. 
A true copy of record, 

Attest, WILLIAM CkAGGETT, Clerk. 






PREFACE. 



■'••©^5© *<*■" 



It is with increased diffidence that the author offers 
this little work to the public. The encouraging recep- 
tion which the Conversations on Chemistry, and 
Pohtical Economy have met with, has induced her 
to venture on publishing a short course on Natural 
Philosophy ; but not without the greatest apprehen- 
sions for its success. Her ignorance of mathematics, 
and the imperfect knov/ledge of natural philosophy 
which that disadvantage necessarily implies, renders 
her fully sensible of her incompetency to treat the 
subject in any other way than in the form of a familiar 
explanation of the first elements, for the use of very 
young pupils. It is the hope of having done this in a 
manner that may engage their attention, which encour- 
ages her to offer them these additional lessons. 

They are intended, in a course of elementary science, 
to precede the Conversations on Chemistry ; and' 
were actually written previous to either of her former 
publications. 

[by the AMERICAN EDITOR.} 

The Conversations on Natural Philosophy, by the 
author of Conversations on Chemistry, are probably 
better adapted to the minds of females and to persons 
generally who are only to acquire the great principles 
of natural science independent of abstruse demonstra- 
tions, than any other treatise pubhshed on tlie subject. 
A persuasion of this, induced the author of the following 
Questions for the examination of scholars in that work, 
to introduce it into the Seminary of which he has the 
superintendence. He soon found, however, that his 
pupils were frequently embarrassed in not knowing, to 
what particular parts they were chiefly to direct the 



iv PREFACE. 

attention, committing to memory what was not neces- 
sary and omitting what was, causing thereby a great 
loss of time as well as of improvement. This led to 
the present system of questions, by the use of which it 
was quickly ascertained, that his pupils could complete 
their lessons in about half the time they before needed, 
and far better than they could before possibly get them. 
And this determined the author to publish and append 
them to the work ; and in coming to the determination, 
it occurred to him, that some additional illustrations 
might advantageousl}^ be introduced, which, together 
with the questions, would form a valuable improvement. 

J. L. BLAKE. 
Concord, N. H. 1823. 



INDEX. 



A. 



Am, 15, 19, 32, 56, 145, 173, 192. 
Air-Pump, 36, 147. 
Angle, 49. 
— , acute, 50. 

, obtuse, 50. 

of incidence, 51, 170, 184. 

of reflection, 51, 163, 169, 

184. 

of vision, 178, 180. 

Aphelion, 82. 
Arctic circle, 99, 1 07 
Atmosphere, 111, 138, 145, 155, 

173. 

, reflection of, 158. 

, color of, 203. 

— — — , refraction of, 190, 

193. 
Attraction, 14, 19, 28, 190. 

■ , of coiiesion, 19, 39, 126, 

145. 

— , of gravitation, 23, 36, 

77, 87, 103, 122, 145. 
Avenue, 180, 
Auditory Nerve, 164. 
Axis, 84. 

of motion, 54, 60. 

of the earth, 99, 106. 

of mirrors, 186. 

- of a lens, 195. 

B. 

Balloon, 35. 
Barometer, 149. 
Bass, 164. 
Bladder, 147. 
Bodies, 14. 

' , elastic, 45, 55. 

, luminous, 166. 

— , sonorous, 160, 164. 

, fall of, 28, 31, 35, 42. 

, opaque, 166, 190. 

-, transparent, 167, 190. 



Camera obscura, 173, 183, 212. 
Capillary tubes, 22. 
Centre, 54. 

of Gravity, 54, 57, 59, 61, 

122. 

»— of motion, 54, 60, 123. 

of magnitude, 54,58. 

Centrifugal force, 55, 79, 102, 122. 

Centripetal force, 55, 79.- 

Ceres, 91. 

Circle, 48, 101, 103, 

Circular motion, 53, 79. 

Clouds, 137. 

Colors, 28, 195. 

Comets, 92. 

Compression, 47. 

Concord, 165. 

Constellation, 92. 

Convergent rays, 185, 187. 

Crystals, 16. 

Cylinder, 58. 



Bulk, 20. 



22 



Day, 84, 112. 

Degrees, 49, 101, 106, 181. 

of latitude, 101, 119. 

of longitude, 101, 119. 

Density, 20. 
Diagonal, 53. 
Diameter, 101. 
Diurnal, 85. 
Discords, 164. 
Divergent rays, 185. 
Divisibility, 14, 16, 

E. 

Earth, 23,77,90, 90,98. 

Echo, 163. 

Eclipse, 117, 120, 168. 

Ecliptic, 93, 100. 

Elastic bodies, 45, 46. 

fluids, 19, 33, 127, 145, 



;!j4 



INDEX. 



Ellipsis, 81. 

Essential properties, 14, 
Exhalations, 17. 
Extension, 14, 15. 
Equator, 99. 
Equinox, 107, 108. 

, precession of, 114. 

Eye, 173. 



Fall of bodies, 28, SI, 35, 42. 
Figure, 14, 16. 
Fluids, 126. 

, elastic, 127, 145. 

, equilibrium of, 128, 150. 

, pressure of, 129, 140, 150. 

Flying, 45. 
Focus, 186. 

' of convex mirrors, 187. 

— of concave, 188. 

• ■ of a lens, 194. 
Force, 38. 

, centrifugal, 55, 79, 102, 122. 

, centripetal, 55, 79. 

— ^ of projection, o6, 78. 

of gravity, 23, 77, 87. Ill, 

145. 
Fountains, 143. 
Friction, 74, 144. 
Frigid zone, 100, 107. 
Fulcrum, 60. 

G. 

General properties of bodies, 14. 
Georgium-Sidus, 91. 
Glass, 194. 

, refraction of, 194. 

— — , burning, 198. 

Gold, 13:3, 

Gravity, 23, 28, 36, 38, 42, 56, 57. 

II. 

ilarmonv, 156. 
Heat, 20, 110. 
Hemisphere, 99, lCt7. 
llvdi'ometer, 136. 
Hydrostatics, 126. 

T. 

lini';;^eo.t tin; retina, 1743 182. 
Image r-:? versed, 1 76. 
Hi i.Iriin mi'iTor. 183. 



' in convex ditto, 185, 
— - in concave, 185. 
Impenetrability, 14. 
Inclined plane, 60,72 
Inertia, 14, 18, 39. 



J. 



Juno, 91. 
Jupiter, 9§, 120. 



Lake, 142. 
Latitude, 101, 119. 
Lens, 194. 

' , convex, 194. 

, concave, 195. 

Lever, 60. 

-, first order, 64. 

' -, second ditto, 66. 

, third ditto, 66. 

Light, 166. 
' ■, pencil of, 167. 

, reflected, 169. 

, of the moon, 171. 

— -, refraction of, 190. 

, absorption of, 198. 

Liquids, 127. 
Longitude, 101, 119. 
Luminous bodies, 166. 
Lunar month, 116. 
- eclipse, 117. 

M. 

Machine, 60, 72, 74. 
Magic Lanthorn, 213. 
jNTars, 90. 
Matter, l4, 44. 
Mechanics, 60. 
Mediums, 167, 190. 
]Melody, 165. 
Mercury planet?, 89, 121. 
Mercury, or quicksilver, 149. 
^feridians, 100. 
^Microscope, 21 1, 214. 
, sins^'le, :^11. 

» solar, 211. 

Minerals, 16. 
Minutes, 101. 
i^ Lou soon s, 157. 
Month, lunar, 116. 
Momcntiini, 43. 63. 
Moon, 80, 87, 91, It B, 121. 



I^'DEX. 



255 



Moon -light, 171 
Motion, 18,58,44, 45. 
— — , uniform, 40. 

, perpetual, 40, 

— — , retarded, 41. 

, accelerated, 41 . 

— — , reflected, 48. 
— — , compound, 52. 
— — , circular, 53, 70. 
, axis of, 54, 61, 

— , centre of, 54, 60, 122. 

— — — , diurnal, 84. 
Musical instruments, 164. 
Mirrors, 183. 
— — — , reflection of, 183. 

, plane or flat, 185, 

, convex, 185. 
■ ■ -, concave, 185, 187. 

—- , axis of, 186. 

— ~, burning, 188. 

N. 

Neap tides, 124. 
Nerves, 175. 

— — , auditory, 164, 175. 
Nerves, optic, 173, 175. 

— ', olfactory, 175. 

Night, 84. 

Nodes, 106, 107, 114. 

O. 

Octave, 165, 

Odour, 17. 

Opaque -bodies, 166, 167. 

Optics, 166. 

Orbit, 89. 

P. 

Pallas, 91. 

Parabola, 57. 

Parallel lines, 30. 

Pellucid bodies, 167. 

Pencil of rays, 167. 

Pendulum, 104. 

Perihelion, 82, 

Perpendicular lines, CO, 49, ICQ. 

Phases, 117. 

Piston, 152. 

Plane, 100. 

Planets, 83, 87, 111. 

Poles, 99. 

Polar star, 107, 119. 

Porosity, 47. 

Powers, meclianical, 60. 

Projection, 56, 78. 



Precession of the equinoxes, 114, 
Pulley, 60, 68. 
Pump, 36, 37, 

, sucking or lifting, 1 53, 

I -, forcing, 153, 155. 
Pupil of the eye, 173. 

R 

Rain, 138. 
Rainbow, 198. 
Rarity, 20. 
Ray of light, 201. 

of reflection, 169. 

of incidence, 170. 

Rays, intersecting, 173. 
Reaction, 44. 
Receiver, 36. 
Reflection of light, 169. 

• -, angle of, 51, 184. 

of mirrors, 183. 

— — - of plain mirrors, 1 85. 
- of convex mirrors, 185. 

of concave mirrors, 185. 

Reflected motion, 48. 
Refraction, 190. 

of the atmosphere, 192, 

i of glass, 194. 

of a lens, 194, 

of a prism, 195, 

Resistance, 60. 
Retina, 173. 

, image on, 174*. 

Rivers, 137. 
Rivulets, 139. 

S. 
Satellites, 87, 1 19, 120. 
Saturn, 91. 
Scales or balance, 61. 
Screw, 60, 73. 
Shadow, 117, 168. 
Siderialtime, 113. 
Sight, 175. 

Signs, Zodiac, 93, 100, 101. 
Smoke, 1 8, 34. 
Solar Microscope, 211. 
Solstice, 106, 107. 
Sound, 159. 
— — , acute, 1 64. 

■ , musical, 164. 
Space, 39. 
Specific £^ravity, 131. 

^ of air, 149 . 

Spectrum, 196. 
Speaking trumpet, 163. 
Sphere, 31, 58, 103. 



256 



INDEX. 



SpringSj 137. 
Springtides, 1-24, 
Square, 88, 92. 
Stars, 83, 93, 112, 119. 
Storms, 156. 
Substance, 14, 
Summer, 82, 106. 
Sun, 77, 87, 166, 192. 
Swimming, 46. 
Syphon, 141. 

T. 

Tangent, 55, 79. 
Telescope, 214. 

■ , reflecting, 214. 

— , refracting, 214. 

Temperate zone, 100, lOS. 
Thermometer, 151. 
Tides, 121. 
— -, neap, 124. 

, spring, 124. 

■— , aerial, 159. 
Time, 112,114. 

, siderial, 115. 

, equal, 115. 

■ , solar, 115. 
Tone, 164. 

Torrid zone, 100, 108, 1S6. 
Transparent bodies, 167. 
Treble and bass, 164. 
Tropics, 99. 

U. 

Undulations, 162. 
Unison, 165. 



Y. 
ValTc, 152. 
Vapor, 21, 34, 137. 
Velocitv, 39, 63. 
Venus, 90, 121. 
Vesta, 91. 
Vibration, 161. 
Vision, 178. 

, angle of, 178. 

, double, 182. 

W. 

Waters, 126, 139. 
— — , spring, 139. 
-, rain, 139. 



-, level of, 128, 133, 137. 



Wedge, 60, 7?,. 
Weight, 20, 28,103, 131, 146, 147. 
Wheel and axle, 60, 71. 
Wind, 155. 

, trade, 156. 

, periodical, 157, 

Winch, 73. 
Winter, 82, 107. 



Year, 112. 

, siderial, 113. 

— , solar, 114. 

Z. 

Zodiac, 93. 
Zone, 100. 

, torrid, 100, 108, 156, 193. 

, temperate, 100, 108. 

, frigid, 100, 107. 



CONTENTS. 

CONVERSATION I. 

On General Properties of Bodies, 

Introdwction ; General Properties of Bodies ; Impenetrability ; 
Extension ; Figure ; Divisibility ; Inertia ; Attraction ; Attraction 
of Cohesion; Density; Rarity; Heat; Attraction of Gravitation. 

Page 13 

CONVERSATION II. 

On the Attraction of Gravity. 

Attraction of Gravitation, continued ; Of Weight ; Of the fall of 
Bodies ; Of the resistance of the Air ; Of the Ascent of Light Bodies. 

Page 27 

CONVERSATION HI. 

On the Laws of Motion. 

Of Motion; Of the Inertia of Bodies; Of Force to Produce Motion; 
Direction of Motion ; Velocity, absolute and relative ; Uniform 
Motion ; Retarded Motion ; Accelerated Motion; Velocity of Fall- 
ing Bodies ; Momentum ; Action and Re-action equal ; Elasticity 
of Bodies; Porosity of Bodies; Reflected Motion; Angles of Inci- 
dence ai}d Pwcflection. Page 3$ 

CONVERSATION IV. 

On Compound Motion. 

Compound Motion the result of two opposite forces ; Of Circular Mo- 
tion, the »-esuit of two forces, one of which confines the body to a 
fixed point ; Centre of motion, the point at rest while the other parts 
of the body move round it; Centre of Magnitude, the middle of a 
body ; Centripetal Force, that which confines a body to a fixed 
central point ; Centrifugal Force, that which impels a body to fly 
from tlie centre ; Fall of Bodies in a Parabola ; Centre of Gravity, 
the Centre of W^eight, or point about which the parts balance each 
other. Page 52 



\J CONTENTS. 

CONVERSATION V. 

On the Mechanical Powers, 

Of the Power of Machines ; Of the Lever in general; Of the Lever of 
the first kind, having the Fulcrum between the Power and the weight ; 
Of the Lever of the second kind, having the weight between the power 
and the Fulcrum; Of the Lever of the third kind, having the power 
between the Fulcrum and the Weight; Of the Pulley ; Of the Wheel 
and Axle ; Of the inclined Plane ; Of the Wedge , Of the Screw. 

Page 60 

CONVERSATION VI. 

ASTRONOMY. 

Causes of the Earth^s Annual Motion. 

©f the Planets, and their motion ; Of the Diurnal motion of the Earth 
and Planets. Page 77 

CONVERSATION VII. 

On the Planets, 

Of the Satellites or Moons; Gravity diminishes as the Square of the 
Distance ; Of the Solar System ; Of Comets; Constellations, signs of 
the Zodiac ; Of Copernicus, Newton, Szc. Page 87 

CONVERSATION VIII. 

On the Earth. 

Of the Terrestrial Globe ; Of the Figure of the Earth ; Of the Pendulum ; 
Of the Variation of the Seasons, and of the Length of Days and 
Nights ; Of the Causes of the Heat of Summer ; Of Solar, Siderial, 
and Equal or Mean Time. Page 9S 

CONVERSATION IX. 

On the Moon. 

Of the Moon's Motion ; Phases of the Moon ; Eclipses of the Moon ; 
Eclipsesof Jupiter's Moons ; Of the Latitude and Longitude ; Of the. 
transits of the inferior Planets ; Of the Tides. Page 116 



CONTENTS* Vll 

CONVERSATION X. 

HYDROSTATICS. 
On the Mechanical Properties of Fluids, 

Definition of a fluid ; Distinction between Fluids and Liquids ; of Non- 
Elastic Fluids, scarcely susceptible of Compression ; Of the Cohesion 
of Fluids ; Of their Gravitation ; Of their Equilibrium ; Of their 
Pressure ; Of Specific Gravity ; Of the Specific Gravity of Bodies 
heavier than Water ; Of those of the same weight as water ; Of those 
lighter than Water ; Of the Specific Gravity of Fluids. Page 126 

CONVERSATION XL 

Of Springs, Fountains, ^c. 

Of the Ascent of Vapor and the Formation of Clouds ; Of the Formation 
and Fall of Rain, kc. ; Of the Formation of Springs ; Of Rivers as.d 
Lakes ; Of Fountains. Page 137 

CONVERSATION XII. 

PNEUMATICS. 
On the Mechanical Properties of Air. 

Of the Spring or Elasticity of the Air ; Of the Weight of the Air ; 
Experiments with the Air Pump ; Of the Barometer ; Mode of 
Weighing Air; Specific Gravity of Air ; Of Pumps ; Description of the 
Sucking Pump; Description of the Forcing Pump. Page 14S 

CONVERSATION XIIL 

On Wind and Sound. 

Of Wind in General ; Of the Trade Wind; Of the Periodical Trade 
Winds; Of the Aerial Tides; Of Sound in General; Of Sonorous 
Bodies ; Of Musical Sounds ; Of Concord or Harmony, and Melody. 

Page 155 

CONVERSATION XIV. 

On Optics. 

Of Luminous, Transparent, and Opaque Bodies; Of the Radiation of 
Light; Of Shadows ; Of the Reflection of Light ; Opaque Bodies seen 
only by Reflected Light ; Vision Explained ; Camera Obscura ; 
Image of Objects on the Retina. Page 166 



viii CONTENTS. 

CONVERSATION XV. 

On the Angle of Vision^ and Reflection of Mirrors. 

Angle of Vision ; Reflection of Plain Mirrors ; Reflection of Convex 
Mirrors ; Reflection of Concave Mirrors, Page 178^ 

CONVERSATION XVI. 

On Refraction and Colors. 

Transmission of Light by Transparent Bodies ; Refraction ; Refraction 
of the Atmosphere; Refraction of a Lens; Refraction of the Prism; 
Of the Colors of Rays of Light ; Of the Colors of Bodies. Page 190 

CONVERSATION XVIL 

OPTICS. 
On the Stimcture of the Eye^ and Optical Instruments. 

Description of the Eye ; Of the Image on the Retina ; Refraction of the 
Humors of the Eye; Of the Use of Spectacles ; Of the Single Micro- 
scope ; Of the Double tVTicroscope ; Of the Solar Microscope ; Magic 
Lanthorn ; Refracting Telebcope; Reflecting Telescope. 

Page 205 



CON VEHSATION 1. 



ON GENERAL PROPERTIES OF BODIES. 

Introduction ; General Properties of Bodies ; Impenetr abil- 
ity ; Extension ; Figure ; Divisihilitij ; Inertia ; At- 
traction ; Attraction of Cohesion ; Density; liarity ; 
Heat ; Attraction of Gravitation. 



EMILY. 

I MUST request your assistance^ my dear Mrs. B. in a charge 
which I have lately undertaken ; it is that of instructing my 
youngest sister, a task, which I find proves more difficult^ than 
I had at first imagined. 1 can teach her the common routine 
of children's lessons tolerably well ; but she is such an inquis- 
itive little creature, that she is not satisfied without an expla- 
nation of every difficulty that occurs to her, and frequently 
asks me questions which I am at a loss to answer. This 
morning, for instance, when I had explained to her that the 
%vorld w^as round like a ball, instead of being fiat as she had 
supposed, and that it w^as surrounded by the air, she asked 
me what supported it. I told her that it required no support ; 
she then enquired why it did not fall as every thing else did ? 
This I confess perplexed me ; for I had myself been satisfied 
w^ith learning that the world floated in the air, without consid- 
ering how unnatural it was that so heavy a body, bearing the 
weight of all otli^r things, should be able to support itself. 

Mrs. B. 1 make no doubt, my dear, but that I shall be 
able to explain this difficulty to you ; but 1 believe that it 
would be almost impossible to render it intelligible to the 
comprehension of so young a child as your sister Sophia. 
You, who are now in your thirteenth year, may, I think, witli 
great propriety, learn not only the cause of this partictdar 
2 



? 



14 GENERAL PROPEKTIES OP BODIES. 

fact, but acquire a general knowledge of the laws by which the 
natural world is governed. 

Emily, Of all things it is what I should most like to learn ; 
but I was afraid it was too difficult a study even at my age. 

Mrs, B, Not when familiarly explained ; if you have 
patience to attend, I will most willingly give you all the infor- 
mation in my power. You may perhaps find the subject 
rather dry at first ; but if I succeed in explaining the laws of 
nature, so as to make you understand them, I am sure that 
you will derive not only instruction, but great amusement 
from that study. 

Emily, I make no doubt of it, Mrs. B. ; and pray begin 
by explaining why the earth requires no support ; for that is 
the point which just now most strongly excites my curiosity. 

I^irs, B, My dear Emily, if I am to attempt to give you a 
general idea of the laws of nature, which is no less than to 
introduce you to a knowledge of the science of natural philos- 
ophy, it will be necessary for us to proceed with some degree 
of regularity . I do not wish to confine you to the systematic 
order of a scientific treatise ; but if we were merely to exam- 
ine every vague question thai may chance to occur, our pro- 
gress would be but very slow. Let us, therefore, begin by 
taking a short survey of the general properties of bodies, 
some of which must necessarily be explained before I can 
attempt to make you understand why the earth requires no 
support. 

When I speak of bodies, I mean substances, of whatever 
nature^ whether solid or fluid ; and matte?' is the general term 
used to denote the substance, w hatever its nature be, of which 
the difierent bodies are composed. Thus, wood is the matter 
of which this table is made ; water is the matter with whicli 
this glass is filled, &c. 

Emily, I am very glad you have explained the meaning 
of the word matter, as it has corrected an erroneous concep- 
tion I had formed of it : I thought that it w^as applicable to 
solid bodies only. 

Mrs, B, There are certain properties which appear to be 
common to all bodies, and are hence called the essential 
properties of bodies ; these are. Impenetrability, Extension, 
Figure, Divisibility, Inertia, and Attraction, These are 
called the general properties of bodies, as we do not suppose 
any body to exist without them. 

By impenetrability y is meant the property which bodies 



GENERAL PROPERTIES OF BODIES. 15 

have of occupying a certain space, so that, where one body is, 
another cannot be, without displacing the former ; for two 
bodies cannot exist in the same place at the same time. A 
liquid may be more easily removed than a solid body ; yet it 
is not the less substantial since it is as impossible for a liquid 
and a sohd to occupy the same space at the same time, as for 
two solid bodies to do so. For instance, if you put a spoon 
into a glass full of water, the water will flow over to make 
room for the spoon. 

Emily, I understand this perfectly. Liquids are in reality 
as substantial or as impenetrable as solid bodies, and they 
appear less so, only because they are more easily displaced. 

Mrs, B, The air is a fluid differing in its nature from 
liquids, but no less impenetrable. If I endeavour, to fill this 
phial by plunging it into this bason of water, the air, you see, 
rushes out of the phial in bubbles, in order to make way for 
the water, for the air and the water cannot exist together in the 
same space, any more than two hard bodies ; and if I reverse 
this goblet, and plunge it perpendicularly into the water, so 
that the air will not be able to escape, the water will no longer 
be able to fill the goblet. 

Emily, But it rises a considerable way into the glass. 

Mrii. B. Because the water compresses or squeezes the air 
into a small space in the upper part of the glass ; but, as long 
as it remains there, no other body can occupy the same place. 

Emily, A difficulty has just occurred to me, with regard to 
the impenetrability of solid bodies ; if a nail is driven into a 
piece of wood, it penetrates it, and both the wood and the nail 
occupy the same space that the wood alone did before ? 

Mrs, B, The nail penetrates between the particles of the 
wood, by forcing them to make way for it ; for you know that 
not a single atom of wood can remain in the space which the 
nail occupies ; and if the wood is not increased in size by the 
addition of the nail, it is because wood is a porous substance, 
like sponge, the particles of which may be compressed or 
squeezed closer together ; and it is thus that they make way 
for the nail. 

We may now proceed to the next general property of bodies, 
extension, A body which occupies a certain space must 
necessarily have extension ; that is to say, lengthy breadth^ 
and de])tli ; these are called the dimensions of extension ; can 
you form an idea of any body without them ? 

Emily, No ; certainly I cannot ; though these dimensions 



lO GENERAL PROPERTIES OF BODIES. 

must, of course, vary extremely in different bodies. The 
length, breadth, and depth, of a box, or of a thimble, are 
very different from those of a walking-stick, or of a hair. 

But is not height also a dimension of extension ? 

Mrs, B. Height and depth are the same dimension, con- 
sidered in different points of view ; if you measure a body, or 
a space, from the top to the bottom, you call it depth ; if from 
1 he bottom upwards, you call it height ; thus the depth and 
height of a box are, in fact, the same thing. 

Emih/. Very true ; a moment's consideration would have 
enabled me to discover that ; and breadth and width are also 
the same dimension. 

Mrs, B, Yes ; the limits of extension constitute^,e?/^*e or 
shape. You conceive that a body having length, breadth, and 
depth, cannot be without form, either symmetrical or irregular ? 

Emily. Undoubtedly ; and this property admits of almost 
an infinite variety. 

ii/rs. B. Nature has assigned regular forms to her pro- 
ductions in general. The natural form of mineral substances 
is that of crystals, of which there is a great variety. Many 
of them are very beautiful, and no less remarkable by their 
transparency, or colour, than by the perfect regularity of their 
forn^Sj-as mav be seen in the various museums and collections 
of natural history. The vegetable and animal creation appears 
less symmetrical, but is still more diversified in figure than the 
mineral kingdom. Manufactured substances assume the vari- 
ous arbitrary forms which the art of man designs for them ; 
and an infinite number of irregular forms are produced by 
fractures, and by the dismemberment of the parts of bodies. 

Emily. Such as a piece of broken china or glass ? 

Mrs. B. Or the fragments of mineral bodies which are 
broken in being dug out of the earth, or decayed by the effect 
of torrents and other causes. The picturesque efiect of rock- 
scenery is in a great measure owing to accidental irregidari- 
ties of this kind. 

We may now proceed to divisihility ; that is to say, a 
susceptibility of being divided into an indefinite number of 
parts. Take any small quantity of matter, a grain of sand 
for instance, and cut it into two parts ; these two parts might 
be again divided, had we instruments sufficient!}'- fine for the 
purpose ; and if, by means of pounding, grinding, and other 
similar methods, we carry this division to the greatest possible 
extent, and reduce the body to its finest imaginable particles^ 



GENERAL PROPERTIES OF BODIES. . 1/ 

yet not one of the particles will be destroyed, and the body 
will continue to exist, though in this altered state. 

The melting of a solid body in a liquid affords a very strik- 
ing example of the extreme divisibility of matter ; when you 
sweeten a cup of tea, for instance, with what minuteness the 
sugar must be divided to be diffused throughout the whole of 
the liquid. 

Emily, And if you pour a few drops of red wine into a. 
glass of water, they immediately tinge the whole of the water, 
and must therefore be diffused throughout it. 

Mrs. B, Exactly so ; and the perfume of this lavender 
water will be almost as instantaneously diffused throughout 
the room, if I take out the stopper. 

Emily, But in this case it is only the perfume of the lav- 
ender, and not the water itself, that is diffused in the room ? 

Mrs, B, The odour or smell of a body is part of the body 
itself, and is produced by very minute particles or exhalations 
which escape from odoriferous bodies. It would be impossi- 
ble that you should smell the lavender water, if particles of it 
did not come in actual contact with your nose. 

Emily, But when I smell a flower, I see no vapour rise 
from it ; and yet I can perceive the smell at a considerable 
distance. 

Mrs, B, You could, I assure you, no more smell a flower, 
the odoriferous particles of which did not touch your nose, 
than you could taste a fruit, the flavoured particles of which 
did not come in contact with your tongue. 

Emily. That is wonderful indeed ; the particles then, 
which exhale from the flower and from the lavender water, 
are, I suppose, too small to be visible ? 

Mrs, B, Certainly : you may form some idea of their 
extreme minuteness from the immense number which must 
have escaped in order to perfume the whole room ; and yet 
there is no sensible diminution of the liquid in the phial. 

Emily. But the quantity must really be diminished ? 

Mrs. B. Undoubtedly ; and were you to leave the bottle 
open a sufficient length of time, the whole of the water would 
evaporate and disappear. But though so minutely subdivided 
as to be imperceptible to any of our senses, each particle 
would continue to exist ; for it is not within the power of 
man to destroy a single particle of matter : nor is there any 
reason to suppose that in nature an atom is ever annihilated, 

Emily, Yet, when a body is burnt to ashes, part of it, at 

a* 



18 GENERAL PROPERTIES OF BODIES. 

leastj appears to be efTectiially destroyed ? Look how small 
is the residue of ashes beneath the grate, from all the coals 
which have been consumed within it. 

Mrs, B. That part of the coals, which you suppose to be 
destroyed, evaporates in the form of smoke and vapour, whilst 
the remainder is reduced to ashes. A body, in burning, 
undergoes no doubt very remarkable changes ; it is generally 
subdivided ; its form and colour altered ; its extension in- 
creased ; but the various parts, into which it has been sepa- 
rated by combustion, continue in existence, and retain all the 
essential properties of bodies. 

Emily, But that part of a burnt body which evaporates 
in smoke has no figure ; smoke, it is true, ascends in columns 
into the air, but it is soon so much diffused as to lose all form ; 
it becomes indeed invisible. 

Mrs. B. Invisible, I allow ; but we must not imagine that 
what we no longer see no longer exists. — Were every particle 
of matter that becomes invisible annihilated, the world itself 
would in the course of time be destroyed. The particles of 
smoke, when diffused in the air, continue still to be particles 
of matter, as well as when more closely united in the form of 
coals : they are really as substantial in the one state as in the 
other, and equally so when by their extreme subdivision they 
become invisible. No particle of matter is ever destroyed : 
this is a principle you must constantly remember. Every 
thing in nature decays and corrupts in the lapse of time. 
We die, and our bodies moulder to dust ; but not a single 
atom of them is lost ; they serve to nourish the earth, whence, 
while living, they drew their support. 

The next essential property of matter is called inertia ; 
this word expresses the resistance which inactive matter makes 
to a change of state. Bodies appear to be equally incapable 
of changing their actual state, whether it be of motion or of 
rest. You know that it requires force to put a body which is 
at rest in motion ; an exertion of strength is also requisite to 
stop a body which is already in motion. The resistance of 
the body to a change of state, in either case, is called its 
inertia, 

Emily, In playing at base-ball I am obliged to use all my 
strength to give a rapid motion to the ball ; and when I have 
to catch it, I am sure I feel the resistance it makes to being 
stopped. But if I did not catch it, it wodd soon fall to the 
ground and stop of itself. 



GENERAL PROPERTIES OF BODIES. 19 

Mrs. B, Inert matter is as incapable of stopping of itself, 
as it is of putting itself into motion : when the ball ceases to 
move, therefore, it must be stopped by some other cause or 
povrer ; but as it is one with which you are yet unacquainted, 
we cannot at present investigate its effects. 

The last property which appears to be common to all bodies 
is attraction. All bodies consist of infinitely small particles 
of matter, each of which possesses the powder of attracting or 
draw^ing towards it, and uniting whh any other particle sufti- 
eiently near to be within the influence of its attraction ; but 
in minute particles this power extends to so very small a 
distance around them that its effect is not sensible, unless they 
are (or at least appear to be) in contact ; it then makes them 
stick or adhere together, and is hence called the attraction of 
cohesion. Without this power, solid bodies would fall in 
pieces or rather crumble to atoms. 

Emily, I am so much accustomed to see bodies firm and 
solid, that it never occurred to me that any power was requi- 
site to unite the particles of which they are composed. But 
the attraction of cohesion does not, I suppose, exist in liquids ; 
for the particles of liquids do not remain together so as to form 
a body, unless confined in a vessel ? 

Mrs, B, I beg your pardon ; it is the attraction of cohe- 
sion which holds this drop of Avater suspended at the end of 
my finger, and keeps the minute watery particles of which it is 
composed united. But as this power is stronger in proportion 
as th3 particles of bodies are more closely united, the cohesive 
attraction of solid bodies is much greater than that of fluids. 

The thmner and lighter a fluid is, the less is the cohesive 
attraction of its particles, because they are further apart ; and 
in elastic fluids, such as air, there is no cohesive attraction 
among the particles. 

Emily, That is very fortunate ; for it would be impossible 
to breathe the air in a solid mass ; or even in a liquid state. 

But is the air a body of the same nature as other bodies ? 

Mrs, B, Undoubtedly, in all essential properties. 

Emily, Yet you say that it does not possess one of the 
general properties of bodies — cohesive attraction ? 

Mrs, B, The particles of air are not destitute of the power 
of attraction, but they are too far distant from each other to 
be influenced by it ; and the utmost eflbrts of human art have 
proved ineffectual in the attempt to compress them, so as tp 
bring them within the sphere of each other's attraction, and 
make them cohere. 



20 GKNERAL PROPERTIES OF BODIES. 

Emily. Ifso, howisit possible to prove that they are 
endowed with this power ? 

Mrs. B. The air is formed of particles precisely of the 
same nature as those which enter into the composition of 
liquid and solid bodies, in which state we have a proof of their 
attraction. 

Emily . It is then, I suppose, owing to the different degrees 
of attraction of different substances, that they are hard or soft ; 
and that liquids are thick or thin ? 

Mrs. B. Yes ; but you would express your meaning better 
by the term dcnsify^ which denotes the degree of closeness 
and compactness of the particles of a body : thus you may 
say, both of solids, and of liquids, that the stronger the cohe- 
sive attraction the greater is the density of the body. In phi- 
losophical language, density is said to be that property of 
bodies by which they contain a certain quantity of matter, 
under a certain bulk or magnitude. Rarity is the contrary 
of density ; it denotes the thinness and subtlety of bodies : 
thus you would say that mercury or quicksilver was a very 
dense fluid ; ether, a very rare one, &c. 

Caroline. But how are we to judge of the quantity of 
matter contained in a certian bulk ? 

Mrs. B. By the weight : under the same bulk bodies are 
said to be dense in proportion as they are heavy. 

Emily. Then we may say that metals are dense bodies, 
wood comparatively a rare one, &c. But, Mrs. B., when the 
particles of a body are so near as to attract each other, the 
effect of this power must increase as they are brought by it 
closer together ; so that one would suppose that the body 
would gradually augment in density, till it was impossible for 
its particles to be more closely united. Now, we know that 
this is not the case ; for soft bodies, such as cork, sponge, or 
butter, never become, in consequence of the increasing attrac- 
tion of their particles, as hard as iron ? 

Mrs. B. In such bodies as cork and sponge, the particles 
which come in contact are so few as to produce but a slight 
degree of cohesion : they are porous bodies, which, owing to 
the peculiar arrangement of their particles, abound with inters- 
tices which separate the particles ; and these vacancies are 
filled with air, the spring or elasticity of which prevents the 
closer union of the parts. But there is another fluid much 
more subtle than air, which pervades all bodies, this is heat. 
Heat insinuates itself more or less between the particles of all 
bodies, and forces them asunder ; you may therefore consider 



i 



OEXERAL PrvOPERTIES OF BODIES. 21 

iit'ai and the attraction of cohesion^ as constantly acting in 
opposition to each other. 

Emily. The one endeavounng to rend a body to pieces, 
the other to keep its parts firmly united. 

Mrs. B. And it is this struggle between the contending 
tbrces of heat and attraction, which prevents the extreme 
degree of density which would result from the sole influence 
of the attraction of cohesion. 

Emihj. The more a body is heated then, the more its 
particles will be separated. 

Mrs. B. Certainly ; we find that bodies swell or dilate 
hy heat : this effect, is very sensible in butter, for instance, 
which expands by the application of heat, till at length the 
attraction of cohesion is so far diminished that the particles 
separate, and the butter becomes liquid. A similar effect is 
produced by heat on metals, and all bodies susceptible of 
being melted. Liquids, you know, are made to boil by the 
application of heat : the attraction of cohesion then yields 
entirely to the expansive powder ; the particles are totally 
separated and converted into steam or vapour. But the 
agency of heat is in no body more sensible than in air, which, 
dilates and contracts by its increase or diminution in a very 
remarkable degree.* 

Emily. The effects of heat appear to be one of the most 
interesting parts of natural philosophy. 

Mrs. B. That is true ; but heat is so intimately connected 
with chemistry, that you must allow me to defer the investi- 
gation of its properties till you become acquainted with that 
science. 

To return to its antagonist, the attraction of cohesion ; it 
is this power which restores to Vctpour its liquid form, which 
unites it into drops when it falls to the earth in a shower of 
rain, which gathers the dQ\Y into brilliant gems on the blades 
of grass. 

Emily. And I have often observed that after a shower, 

* The expansive power of heat proiluces some of the most iiuerestingf 
phenomena in nature. Tlie hoiling of liquids, is the immediate result of 
this power; and the operation, although simple, is peculiarly worthy of 
notice. As the numerous particles hecome expanded or rarified, they 
are continually rising to, and escaping from the surface, which occasions 
an agitation of the liquid, proportioned, in its violence, to the degree of 
heat operating on it. — And on exposing our hands or other limbs to the 
fire, the internal fluid becomes expanded, "which causes them to ap])ear 
swollen ; whereas, when exposed to the cold, the abstraction of the heat 
causes them to be compressed. 



'22 GENERAL PROPERTIES OF BODIES. 

1 he water collects into large drops on the leaves of plants ; 
but I cannot say that I perfectly understand how the attrac- 
tion of cohesion produces this effect. 

Mrs, B, Rain does not fall from the clouds in the form of 
drops J but in that of mist or vapour, which is composed of 
very small watery particles ; these in their descent, mutually 
attract each other, and those that are sufficiently near in con- 
sequence unite and form a drop, and thus the mist is trans- 
formed into a shower. The dew also was originally in a 
state of vapour, but is, by the mutual attraction of the 
particles, formed into small globules on the blades of grass : 
in a similar manner the ram upon the leaf collects into large 
drops, v/hlch when they become too heavy for the leaf to 
support, fall to the ground. 

Emily. All this is wonderfully curious ! I am almost 
bewildered with surprise and admiration at the number of new 
ideas I have already acquired. 

Mrs, jB. Every step that you advance in the pursuit of 
natural science, will fill your mind with admiration and grat- 
itude towards its Divine Author. In the study of natural 
philosophy, we must consider ourselves as reading the book 
of nature, in which the bountiful goodness and wisdom of God 
is revealed to all mankind ; no study can then tend more to 
purify the heart, and raise it to a religious contemplation of 
the Divine perfections. 

There is another curious effect of the attraction of cohesion 
which I must point out to you. It enables liquids to rise 
above their level in capillary tubes ; these are tubes, the bores 
of which are so extremely small that liquids ascend within 
them, from the cohesive attraction between the particles of 
the liquid and the interior surface of the tube. Do you 
perceive the water rising above its level in this small glass 
tube, which I have immersed in a goblet full of water ? 

Emihj. Oh yes ; I see it slowly creeping up the tube, but 
now it is stationary ; will it rise no higher ? 

Mrs. I>. No ; because the cohesive attraction between 
the water and the internal surface of the tube is now balanced 
by the weight of the water within it : if the bore of the tube 
were narrower the water would rise higher ; and if you im- 
merse several tubes of bores of different sizes, you will see it 
rise to different heights in each of them. In making this 
experiment, you should colour the water w ith a little red wine, 
in order to render the effect more obvious. 

All porous substances, such as sponge, bread, linen, &c 



GENERAL PROPERTIES OF BODIES. 2S 

may be considered as collections of capillary tubes : if you 
dip one end of a lump of sugar into water the water will rise 
in it ; and wet it considerably above the surface of that into 
which you dip it. 

" EmiJt/. In making tea I have often observed that effectj 
without being able to account for it. 

Mrs, B. Now that you are acquainted with the attraction 
of cohesion, I must endeavor to explain to you that of Grav- 
itation, which is a modification of the same power ; the first 
is perceptible only in very minute particles, and at very small 
distances ; the other acts on the largest bodies, and extends 
to immense distances. 

Emily. You astonish me : surely you do not mean to say 
that large bodies attract each other. 

Mrs, B. Indeed I do ; let us take, for example, one of the 
largest bodies in nature, and observe whether it does not 
attract other bodies. What is it that occasions the fall of this 
book, when I no longer support it ? 

Emily, Can it be the attraction of the earth ? I thought 
that all bodies had a natural tendency to fall. 

Mrs, B, They have a natural tendency to fall, it is true ; 
but that tendency is produced entirely by the attraction of the 
earth ; the earth being so much larger than any body on 
its surface, forces every body, wliich is not supported, to fall 
upon it. 

Emily. If the tendency which bodies have to fall results 
from the earth's attractive power, the earth itself can have no 
such tendency, since it cannot attract itself, and therefore it 
requires no support to prevent it from frilling. Yet the idea 
that bodies do not fall of their own accord, but that they are 
drawn towards the earth by its attraction, is so new and 
strange to me, that I know not how to reconcile myself to it. 

Mrs, B, When you are accustomed to consider the fall of 
bodies as depending on this cause, it will appear to you as 
natural, and surely much more satisfactory, than if the cause 
of their tendency to fall were totally unknown. Thus you 
understand, that all matter is attractive, from the smallest 
particle to the largest mass ; and that bodies attract each 
other with a force proportional to the quantity of matter they 
contain. 

Emily, I do not perceive any difference between the 
attraction of cohesion and that of gravitation : is it not because 
every particle of matter is endowed with an attractive power, 



24 f.i\St.AAL rKOPElLiii:.;^ Oi i;oDiJb.>. 

that large bodies^ consisting of a great number oi" paniciesj 
are so strongly attractive ? 

Mrs. B. True. There is, however, this difference be- 
tween the attraction of particles and that of masses, that the 
former is stronger than the latter, in proportion to the quan- 
tity of matter. Of this you have an instance in the attraction 
of capillary tubes, in which liquids ascend by the attraction of 
cohesion, in opposition to that of gravity. It is on this ac- 
count that it is necessary that the bore of the tube should be 
eitremely small ; for if the column of water within the tube is 
not very minute, the attraction would not be able either to 
raise or support its weight, in opposition to that of gravity. 

You may observe, also, that all solid bodies are enabled by 
the force of the cohesive attraction of their particles to resist 
that of gravity, which would otherwise disunite them, and 
bring them to a level with the ground, as it does in the case 
of liquids, the cohesive attraction of w^iich is not sufficient to 
enable it to resist the powTr of gravity.* 

Emily, And some solid bodies appear to be of this n^l- 
ture, as sand and powder for instance : there is no attraction 
of cohesion between their particles ? 

* The pov/er of gravitation is greatest at the surface of the earth, -v^'hence 
it decreases both upwards and downwards ; but not in the same propor- 
tion. The force of gravity upxvards is as the square of the distance from 
the centre. That is, gravity at the surface of the earth, which is about 
4300 miles from the centre, is four times more powerful than it -would 
be at double the distance, or 8000 miles from tlie centre. Gravity and 
■weight may be taken, in particular circumstances as synonymous terms. 
We say, a piece of lead weighs a pound, or sixteen ounces ; but if by any 
TAieans it could be carried 4000 miles above the surface of the earth, it 
vvould weigh only one fourth of a pound, or four ounces ; and if it could 
be transported to 8000 miles above the earth, which is three times the 
distance from the centre that the surface is, it would w^eigh only one ninth 
of a pound, or something less than two ounces. 

And it is demonstrated, that the force of gravity downwards decreases, 
as the distance from the surface increases, so that at one half the distance 
from the centre to the surface, the same weight, already described would 
-weigh only one hail of a pound, and so on. — Thus, a x>iece of metal weigh- 
uig, on the surface of the earth, one pound, will 

At The centre weigh - - - - 

1000 miles from the centre 1-4 pound. 

2000 1-2 

5000 3-4 

4000 1 

8000 1-4 

12,000 1-9 

And at the distance of the moon from the earth, Avhich is 240,000 miles, 
it would weigh oidy the 3,600th part of a pound, because the distance is 
60 times further from the centre of the earth than the surface-. 



GE^[ERAL PKOPERTIES OF BODIES. ' 25 

Mrs, B. Every grain of powder or sand is composed of a 
great number of other more minute particles, firmly united by 
the attraction of cohesion ; but amongst the separate grains 
tliere is no sensible attraction, because they are not in suffi- 
ciently close contact. 

Emily, Yet they actually touch each other ? 

Mrs, B. The surface of bodies is in general so rough and 
uneven, that when in actual contact, they touch each other 
only by a few points. Thus, if I lay upon the table this 
book, the binding of which appears perfectly smooth, yet so 
few of the particles of its under surface come in contact with 
the table, that no sensible degree of cohesive attraction takes 
place ; for you see, that it does not stick, or cohere to the 
table, and I find no difficuhy in lifting it off. 

It is only when surfaces perfectly flat and well polished 
are placed in conta<:t, that tlie particles approach in sufficient 
number, and closely enough, to produce a sensible degree of 
cohesive attraction. Here are two hemispheres of polished 
metal, I press their flat surfaces together, having previously 
interposed a few drops of oil, to fill up every little porous 
vacancy. Now try to separate them. 

Emily, It requires an effort beyond my strength, though 
there are handles for the purpose of pulling them asunder. 
Is the firm adhesion of the two hemispheres, merely owing to 
the attraction of cohesion ? 

Mrs, B, There is no force more powerful, since it is by 
this that the particles of the hardest bodies are held together, 
it would require a weight of several pounds, to separate these 
hemispheres. 

Emily, In making a kaleidoscope, I recollect that the two 
plates of glass, which were to serve as mirrors, stuck so fast 
together, that I imagined some of the gum I had been using 
had by chance been interposed betv/een them ; but now I 
make no doubt but that it was their own natural cohesive 
attraction which produced this effect. 

Mrs B, Very probably it was so ; for plate-glass has an 
extremely smooth, flat surface, admitting of the contact of a 
great number of particles, between two plates, laid one over 
the other. 

Emily, But, Mrs. B., the cohesive attraction of some 
bodies is much greater than that of others ; thus glue, gum, 
and paste, cohere with singular tenacity. 
3 



Ju GENERAL PROPERTIES OF BODIES. 

Mrs, B. That is owing to the peculiar chemical properties 
of those bodies, independently of their cohesive attraction. 

There are some other kinds of modifications of attraction 
peculiar to certain bodies ; namely, that of magnetism, and 
of electricity ; but we shall confine our attention merely to 
the attraction of cohesion and of gravity ; the examination of 
the latter we shall resume at our next meeting. 



CONVERSAnON II. 



ON THE ATTRACTION OF GRAVITY. 

Attraction of Gramtation, continued ; of Weight ; Of the 
fall of Bodies ; Of the Resistance of the Air ; Of the 
Ascent of Light Bodies, 



EMILY. 

I HAVE related to my sister Caroline all that you have 
taught me of natural philosophy, and she has been so much 
delighted by it, that she hopes you will have the goodness to 
admit her to your lessons. 

Mrs. B, Very willingly ; but I did not think you had 
any taste for studies of this nature, Caroline ? 

Caroline, I confess, Mrs. B., that hitherto I had formed 
no very agreeable idea, either of philosophy, or philosophers ; 
but what Emily has told me, has excited my curiosity so 
much, that I shall be highly pleased if you will allow me to 
become one of your pupils. 

Mrs. B. I fear that I shall not find you so tractable a 
scholar as Emily ; I know that you are much biassed in 
favour of your own opinions. 

Caroline. Then you will have the greater merit in re- 
forming them, Mrs. B. ; and after all the wonders that Emily 
has related to me, I think I stand but little chance against 
you and your attractions. 

Mrs. B. You will, I doubt not, advance a number of 
objections ; but these I shall willingly admit, as they will be 
a means of elucidating the subject. Emily, do you recollect 
the names of the general properties of bodies ? 

Emily. Impenetrability, extension, figure, divisibility, 
inertia, and attraction. 



^b OS THE ATTRACTION OF GRAVITY. 

Mrs. B, Very well. You must remember that these arf 
properties common to all bodies, and of which they cannot 
be deprived ; all other properties of bodies are called acci- 
ilental, because they depend on the relation or connection of 
one body to another. 

Caroluic, Yet surely, Mrs. B., there are other properties 
which are essential to bodies, besides those you have enumer- 
ated. Colour and weight, for instance, are common to ail 
bodies, and do not arise from their connexion with each other, 
but exist in the bodies themselves ; these, therefore, cannot 
be accidental qualities. 

Mrs, B. I beg your pardon ; these properties do not exist 
in bodies independently of their connection with other bodies. 

Caroline, What ! have bodies no weight ? Does not this 
table weigh heavier than this book ; and, if one thing weighs 
iieavier than another, must there not be such a thing as 
weight ? 

Mrs, B, No doubt : but this property does not appear to be 
essential to bodies ; it depends upon their connection with each 
other. AVeight is an effect of the power of attraction, without 
which the table and the book would have no weight whatever. 

Emily. I think I understand you : is it not the attraction 
of gravity, which makes bodies heavy ? 

Mrs, B. You are right. I told you that the attraction of 
gravity v/ as proportioned to the quantity of matter which 
bodies contained : now the earth consisting of a much greater 
quantity of matter than any body upon its surface, the force 
of its attraction must necessarily be greatest, and must draw 
every thing tovrards it ; in consequence of which, bodies that 
are unsupported fall to the ground, w^hilst those that arc sup- 
[)orted press upon the object which prevents their fall, v/ith a 
weight equal to the force with which they gravitate towards 
die earth. 

Caroline, The same cause then which occasions the fall 
of bodies, produces also their weight. It was very dull in me 
not to understand this before, as it is the natural and necessary 
consequence of attraction ; but the idea that bodies were not 
really heavy of themselves, appeared to me quite incompre- 
hensible. But, Mrs. B., if attraction is a property essential to 
matter, weight must be so likewise ; for how can one exist 
without the other ? 

Mrs, B, Suppose there were but one body existing in 
universal space, what would its weight be ? 



ON THE ATTRACTION OF GRAVITY* 29 

Caroline. That would depend upon its size ; or, more 
accurately speaking, upon the quantity of matter it contained. 

Emily, No, no ; the body would have no weight whatever 
were its size ; because nothing would attract it. Am I not 
right, Mrs. B. ? 

Mrs. B. You are : you must allow, therefore, that it would 
be possible for attraction to exist without weight ; for each of 
the particles of which the body was composed, would possess 
the power of attraction ; but they could exert it only amongst 
themselves ; the whole mass, having nothing to attract, or to 
be attracted by, would have no weight. 

Caroline. I am now well satisfied that weight is not 
essential to the existence of bodies ; but what have you to 
object to colours, Mrs. B., you will not, I think, deny that 
they really exist in the bodies themselves. 

Mrs. B. When we come to treat of the subject of colours, 
I trust that I shall be able to convince you, that colours are 
likewise accidental qualities, quite distinct from the bodies to 
which they appear to belong. 

Caroline. Oh do pray explain it to us now, I am so very 
curious to know how that is possible. 

Mrs. B. Unless we proceed with some degree of order 
and method, you will in the end find yourself but little the 
wiser for all you learn. Let us therefore go on regularly, and 
make ourselves well acquainted with the general properties 
of bodies, before we proceed further. 

Emily. To return, then, to attraction, (which appears to 
me by far the most interesting of them, since it belongs equal- 
ly to all kinds of matter,) it must be mutual between two 
bodies ; and if so, when a stone falls to the earth, the earth 
should rise part of the way to meet the stone ? 

Mrs. B. Certainly ; but you must recollect that the force 
of attraction is proportioned to the quantity of matter which 
bodies contain, and if you consider the difference there is in 
that respect, between a stone and the earth, you will not be 
surprised that you do not perceive the earth rise to meet the 
stone ; for though it is true that a mutual attraction takes 
place between the earth and the stone, that of the latter is so 
very small in comparison to that of the former, as to render 
its effect insensible., 

Emily. But since attraction is proportioned to the quantity 
of matter which bodies contain, why do not the hills attract 
the houses aad churches towards them ? 



30 ON THE ATTRACTION OP GRAVITY. 

Caroline, Heavens, Emily, what an idea ! Hov/ can the 
houses and churches be moved, when they are so firmly fixed 
in tlie ground ? 

Mrs. B. Emily's question is not absurd, and your answer, 
Caroline, is perfectly just ; but can you tell us why the houses 
und churches are so firmly fixed in the ground ? 

Caroline, I am afraid I have answered risfht by mere 
chance ; for I begin to suspect that bricklayers and carpenters 
itould give but little stability to their buildings, without the aid 
of attraction. 

Mrs, B, It is certainly the cohesive attraction between the 
bricks and the mortar, which enables them to build walls, and 
these are so strongly attracted by the earthy as to resist every 
other impulse ; otherwise they would necessarily move towards 
the hills and the mountains ; but the lesser force must yield to 
the greater. There are, however, som_e circumstances in 
which the attraction of a large body has sensibly counteracted 
that of the earth. If, whilst standing on the declivity of a 
mountain, you hold a plumb-line in your hand, the weight will 
not fall perpendicular to the earth, but incline a httle towards 
the mountain ; and this is owing to the lateral, or sideways 
attraction of the mountain, interfering with the perpendicular 
attraction, of the earth. 

Emily, But the size of a mountain is very trifling compared 
ro the whole earth ? 

Mrs, B, Attraction, you must recollect, diminishes w^itli 
distance ; and in the example of the plumb-line, the weight 
suspended is considerably nearer to the mountain than to the 
centre of the earth. 

Caroline, Pray, Mrs. B.,^ do the two scales of a balance 
hang parallel to each other ? 

Mrs, B, You mean, I suppose, in other words, to inquire 
Avhether two lines which are perpendicular to the earth, are 
parallel to each other ? I believe I guess the reason of your 
question ; but I wish you w ould endeavour to answ^er it without 
my assistance. 

Caroline, I was thinking that such lines must both tend 
by gravity to the same point, the centre of the earth ; now 
lines tending to the same point cannot be parallel, as parallel 
lines are always at an equal distance from each other, and 
would never meet. 

Mrs, B, Very well explained ; you see now the use of your 
knowledge of parallel lines : had you been ignorant of theio; 



TLATE I 




ON THE ATtRACTION OF GP.AVITr. Si 

properties, you couid not have drawn such a conclusion. This 
may enable you to form an idea of the great advantage to be 
derived even from a slight knowledge of geometry, in the study 
of natural philosophy ; and if, after I have made you acquain- 
ted with the first elements, you should be tempted to pursue 
the study, I would advise you to prepare yourselves by acqui- 
ring some knovvdedge of geometry. This science v/ould teach 
you that lines which fall perpendicular to the surface of a 
sphere cannot be parallel, because they would all meet, if 
prolonged to the centre of the sphere ; while lines that fail 
perpendicular to a plane or flat surface, are always parallel, 
because if prolonged, they would never meet. 

Emily, And yet a pair of scales, hanging perpendicular 
to the earth, appear parallel ? 

Mrs. B, Because the sphere is so large, and the scales 
consequently converge so little, that their inclination is not 
perceptible to our senses ; if we could construct a pair of 
scales whose beam would extend several degrees, their con- 
vergence would be very obvious ; but as this cannot be accom- 
plished, let us draw a small figure of the earth, and then we 
may make a pair of scales of the proportion we please, 
(fig. 1. plate I.) 

Caroline, This figure renders it very clear : then two 
bodies cannot fall to the earth in parallel lines ? 

Mrs. B. Never. 

Caroline, The reason that a heavy body falls quicker than 
a hght one, is, I suppose, because the earth attracts it more 
strongly ? 

Mrs. B, The earth, it is true, attracts a heavy body more 
than a light one ; but that would not make the one fall quicker 
than the other. 

Caroline, Yet, since it is attraction that occasions the fall 
of bodies, surely the more a body is attracted, the more rapid- 
ly it will fall. Besides, experience proves it to be so. Do 
we not every day see heavy bodies fall quickly, and light 
bodies slowly ? 

Emily. It strikes me, as it does Caroline, that as attraction 
is proportioned to the quantity of matter, the earth must 
necessarily attract a body which contains a great quantity 
more strongly, and therefore bring it to the ground sooner than 
one consisting of a smaller quantity. 

Mrs, B. You must consider, that if heavy bodies are 
attracted more strongly than Hght ones, they require more 



32 ON THE ATTRACTION OF GRAVITY, 

attraction to malie thcin fall. Remember that bodies have no 
natural tendency to faH, any more than to rise, or to move 
laterally, and that they will not fall unless impelled by some 
force ; now this force must be proportioned to the quantity of 
matter it has to move : a body consisting of 1000 particles of 
matter, for instance, requires ten times as much attraction to 
bring it to the ground in the same space of time as a body 
consisting of only 100 particles. 

Caroline. I do not understand that ; for it seems to me, 
that the heavier a body is, the more easily and readily it falls. 

Emily, I think I nov/ comprehend it ; let me try if I can 
explain it to Caroline. Suppose that I draw^ towards me tv/o 
weighty bodies, the one of lOOlbs., the other of lOOOlbs.^ must 
I not exert ten times as much strength to draw the larger one 
to me, in the same space of time as is required for the smaller 
one ? And if the earth draws a body of lOOOlbs. weight to it 
in the same space of time that it draws a body of lOOlbs. does 
li not follow that it attracts the body of lOOOlbs. weight with 
ten times the force that it does that of lOOlbs. ? 

CaroUrie, I comprehend your reasoning perfectly ; but if 
it were so, the body of lOOOlbs. weight, and that of lOOlbs. 
Vv'ouldfall with the same rapidity ; and the consequence would 
be, that all bodies, whether light or heavy, being at an equal 
distance from the ground, would fall to it in the same space of 
time : now it is very evident that this conclusion is absurd ; 
experience cver}^ instant contradicts it ; observe how much 
sooner this book reaches the floor than this sheet of paper, 
when I let them drop together. 

Emily. That is an objection I cannot answer. I must 
refer it to you, Mrs. B. 

Mrs, B. I trust that v/e shall not find it insurmountable, 
\t is true that, according to the laws of attraction, all bodies- 
at an equal distance from the earth, should fall to it in the same 
space of time ; and this would actually take place if no ob- 
stacle intervened to impede their fall. But bodies fall through 
the air, and it is the resistance of the air which makes bodies 
of different density fall with different degrees of velocity. 
They must all force their way through the air, but dense heavy- 
bodies overcome this obstacle more easily than rarer and 
lighter ones> 

The resistance which the air opposes to the fall of bodies 
is proportioned to their surface, not to their weight ; the air 
b^ing inert^ cannot exert a greater foice to support the weight 



ON THE ATTE ACTION OF GRAVITY. SS 

of a cannon-ballj than it does to support the weight of a ball 
(of the same size) made of leather ; but the cannon-ball will 
overcome this resistance more easily^ and fall to the ground, 
consequently, quicker than the leather ball. 

Caroline. This is very clear, and solves the difficulty 
perfectly. The air offers the same resistance to a bit of lead 
and a bit of feather of the same size ; yet the one seems to 
meet with no obstruction in its fall, whilst the other is 
evidently resisted and supported for some time by the air. 

Emily, The larger the surface of a body, then, the more 
air it covers, and the greater is the resistance it meets with 
from it. 

Mrs. B. Certainly : observe the manner in which this 
sheet of paper falls ; it floats awhile in the air, and then 
gently descends to the ground. I will roll the same piece of 
paper up into a ball : it offers now but a small surface to the 
air, and encounters therefore but little resistance : see how 
much more rapidly it falls. 

The heaviest bodies may be made to float awhile in the air, 
by making the extent of their surface counterbalance their 
weight. Here is some gold, which is the most dense body 
we are acquainted with, but it has been beaten into a very 
thin leaf, and offers so great an extent of surface in proportion 
to its weight, that its fall, you see, is still more retarded by 
the resistance of the air than that of the sheet of paper. 

Caroline. That is very curious ; and it is, I suppose, upon 
the same principle that iron boats may be made to float on 
water ? 

But, Mrs. B., if the air is a real body, is it not also subjec- 
ted to the laws of gravity ? 

Mrs. B. Undoubtedly: 

Caroline. Then v/hy does it not, like all other bodies, 
fall to the ground ? 

Mi^s. B. On account of its spring or elasticity. The air 
is an elastic fluid ; a species of bodies, the peculiar property 
of which is to resume, after compression, their original dimen- 
sions ; and you must consider the air of which the atmosphere 
is composed as existing in a state of compression, for its par- 
ticles being drawn towards the earth by gravity, are brought 
closer together than they would otherwise be, but the spring 
or elasticity of the air by which it endeavours to resist com- 
pression gives it a constant tendency to expand itself, so as 
ro resimi^^ the dimensions it would naturally have, if not 



34 ON THE ATTRACTION OF GRAVITY. 

under the influence of gravity. The air may therefore be said 
constantly to struggle with the power of gravity without 
being abie to overcome it. Gravity thus confines the air to 
the regions of our globe, w^hilst its elasticity prevents it from, 
falling like other bodies to the ground. 

Emily, The air then is I suppose, thicker, or I should 
rather say more dense, near the surface of the earth, than in 
the higher regions of the atmosphere ; for that jiart of the air 
which is nearer the surface of the earth must be most strongly 
attracted. 

Mrs, B, The diminution of the force of gravity, at so 
small a distance as that to which the atmosphere extends 
(compared with the size of the earth) is so inconsiderable as 
to be scarcely sensible ; but the pressure of the upper parts 
of the atmosphere on those beneath, renders the air near the 
surface of the earth much more dense than the upper regions. 
The pressm'e of the atmosphere has been compared to that of 
a pile of fleeces of wool, in which the lower fleeces are pressed 
together by the weight of those above ; these lie light and 
loose, in proportion as they approach the uppermost fleece, 
which receives no external pressure, and is confined merely 
by the force of its ow n gravity. 

Caroline, It has just occurred to me that there are some 
bodies w^hich do not gravita,te tov/a^'ds the earth. Smoke and. 
steam, for instance, rise instead of falling. 

Mrs, B, It is still gravity which produces their ascent ; 
at least, were that power destroyed, these bodies would not rise. 

Caroline, I shall be out of conceit with gravity, if it is so 
inconsistent in its operations. 

Mrs. B. There is no difficulty in reconciling this apparent 
inconsistency of eflect. The air near the earth is heavier 
than smoke, steam or other vapours ; it consequently not 
only suppoits these light bodies, but forces them to rise, till 
they reach a part of the atmosphere, the weight of w^hich is 
not greater than their own, and then they remain stationary. 
Look at this basin of water : why does the piece of paper 
which I throw into it float on the surface ? 

Emihj, Because, being lighter than the water, it is sup- 
ported by it. 

Mrs, B. And now that I pour more water into the basin, 
why does the paper rise ? 

Emihj, The water being heavier tlian the paper, gets 
beaeatli it, and obliges it to rise. 



0>» THE ATTRACTION OF GRAVITY. Sb 

Mrs. B. In a similar manner are smoke and vapour forced 
upwards by the air ; but these bodies do not, hke the paper, 
ascend to the surface of the fluid, because, as we observed 
before, the air being thinner and lighter as it is more distant 
from the earth, vapours rise only till they attain a region of 
air of their own density. Smoke, indeed, ascends but a very 
little way ; it consists of minute particles of fuel carried up 
by a current of heated air from the fire below : heat, you 
recollect, expands all bodies ; it consequently rarefies air, and 
renders it lighter than the colder air of the atmosphere ; the 
heated air from the fire carries up with it vapour and small 
particles of the combustible materials which are burning in 
the fire. When ih.s current of hot air is cooled by mixing 
with that of the atmosphere, the minute particles of coal or 
other combustible fall, and it is this which produces the small 
black flakes which render the air and every thing in contact 
with it, in London, so dirty. 

Caroline. You must, however, allow me to make one 
more objection to the universal gravity of bodies ; which is 
the ascent of air balloons, the materials of which are un- 
doubtedly heavier than air ; how, therefore, can they be 
supported by it ? 

Mrs.B. I admit that the materials of which balloons are 
made are heavier than the air ; but the air with which they 
are filled is an elastic fluid, of a different nature from the 
atmospheric air, and considerably lighter ; so that on the 
whole, the balloon is lighter than the air which it displaces, 
and consequently will rise, on the same principle as smoke 
and vapour. Now, Emily, let me hear if you can explain 
how the gravity of bodies is modified by the effect of the air ? 

Emily. The air forces bodies which are lighter than itself 
to ascend ; those that are of an equal weight will remain 
stationary in it ; and those that are heavier will descend 
through it : but the air will have some effect on these last ; 
for if they are not much heavier, they will with difficulty 
overcome the resistance they meet with in passing through it, 
they will be borne up by it, and their fall will be more or less 
retarded. 

Mrs. B. Very well. Observe how slowly this light 
feather falls to the ground, while a heavier body, like this 
marble, overcomes the resistance which the air makes to its 
descent much more easily, and its fall is proportionally more 
rapid. I now throw a pebble into this tub of water ; it does 



S6 ON THE ATTRACTION OF GRAVITV. 

not reach the bottom near so soon as if there were no water in 
the tub, because it meets with resistance from the water. 
Suppose that we could empty the tub, not only of water, but 
of air also, the pebble would then fall quicker still, as it would 
in that case meet with no resistance at all to counteract its 
gravity. 

Thus you see that it is not the different degrees of gravity, 
but the resistance of the air, which prevents bodies of different 
weight from falling with equal velocities ; if the air did not 
bear up the feather, it would reach the ground as soon as the 
marble. 

Ccu'oline, I make no doubt that it is so ; and yet I do not 
feel quite satisfied. I wish there was any place void of air, 
in which the experiment could be made. 

Mrs, B. If that proof will satisfy your doubts, I can give 
it you. Here is a machine called an air pump^{^g, 2. pi. I.) 
by means of which the air may be expelled from any close 
vessel which is placed over this opening, through which the 
air is pumped out. Glasses of various shapes, usually called 
receivers, are employed for this purpose. We shall now 
exhaust the air from this tall receiver which is placed over the 
opening, and we shall find that bodies of whatever weight or 
size within it, will fall from the top to the bottom in the same 
space of time. 

Caroline. Oh, I shall be delighted with this experiment ; 
what a curious machine ! how can you put the two bodies of 
different weight within the glass, without admitting the air. 

Mrs, B, A guinea and a feather are already placed there 
for the purpose of the experiment : here is, you see, a contri- 
vance to fasten them in the upper part of the glass ; as soon 
as the air is pumped out, I shall turn this little screw, by 
which means the brass plates which support them will be 
inclined, and the two bodies will fall. — Now I believe I have 
pretty well exhausted the air. 

Caroline, Pray let me turn the screw. I declare, they 
both reached the bottom at the same instant ! Did you see, 
Emily, the feather appeared. as heavy as the guinea ? 

Emily, Exactly ; and fell just as quickly. How won- 
derful this is ! what a number of entertaining experiments 
might be made with this machine ! 

Mrs, B, No doubt there are a great variety ; but we shall 
reserve them to elucidate the subjects to which they relate : 
if I had not explained to you why the guinea and the feather 



ON THE ATTRACTION OF GRAVITY. 37 

ieil with equal velocity, you would not have been so well 
pleased with the experiment. 

Emily, I should have been as much surprised, but not so 
much interested ; besides, experiments help to imprint on the 
memory the facts they are intended to illustrate ; it will be 
better therefore for us to restrain our curiosity, and wait for 
other experiments in their proper places. 

Caroline, Pray by what means is the air exhausted in this 
receiver. 

Mrs. B, You must learn something of mechanics in order 
to understand the construction of a pump. At our next meet- 
ing, therefore, I shall endeavour to make you acquainted with 
the laws of motion, as an introduction to that subject. 



CONVERSATION III. 



ON THE LAWS OF MOTION. 

On Motion ; Of the Inertia of Bodies ; Of Force to Produce 
Motion ; Direction of Motion ; Velocity, Absolute and 
Relative ; Uniform Motion ; Retarded Motion ; Accele- 
rated Motion ; Velocity of Falling Bodies ; Momentum ; 
Action and Re-action Equal ; Elasticity of Bodies ; 
Porosity of Bodies ; Refected Motion y- Angles of Inci- 
dence and Reflection, 



MRS. B. 

The science of mechanics is founded on the laws of motion ; 
it will, therefore, be necessary to make you acquainted with 
these laws before we examine the mechanical powers. Tell 
me, Caroline, what do you understand by the word motion ? 

Qftroline. 1 think I understand it perfectly, though I am 
at a loss to describe it. Motion is the act of moving about, 
going from one place to another, it is the contrary of remain- 
ing at rest. 

Mrs, B. Very well. Motion then consists in a change of 
place ; a body is in motion whenever it is changing its situa- 
tion with regard to a fixed point. 

Now since we have observed that one of the general pro- 
perties of bodies is Inertia, that is, an entire passiveness either 
with regard to motion or rest, it follows that a body cannot 
move without being put into motion ; the power which puts a 
body into motion is called force ; thus the stroke of the 
hammer is the force which drives the nail ; the pulling of the 
horse that which draws the carriage, &c. Force then is the 
cause which produces motion. 

Emily, And may we not say that gravity is the force 
which occasions the fall of bodies ? 



ON THE LAWS OP MOTION. S\) 

Mrs. B. Undoubtedly. I had given you the most familiar 
illustrations in order to render the explanation clear ; but 
since you seek for more scientific examples, you may say that 
cohesion is the force which binds the particles of bodies to- 
gether, and heat that which drives them asunder. 

The motion of a body acted upon by a single force is always 
in a straight line, in the direction in which it received the 
impulse. 

Caroline. That is very natural ; for as the body is inert, 
and can move only because it is impelled, it will move only 
in the direction in wiiich it is impelled. The degree of quick- 
ness wath which it moves, must, I suppose, also depend upon 
the degree of force with which it is impelled. 

Mrs. B. Yes ; the rate at which a body moves, or the 
shortness of the time which it takes to move from one place 
to another, is called its velocity ; and it is one of the laws of 
motion that the velocity of the moving body is proportional 
to the force by which it is put in motion. We must distin- 
guish between absolute and relative velocity. 

The velocity of a body is called absolute^ if we consider 
the motion of the body in space, without any reference to that 
of other bodies. When for instance a horse goes fifty miles 
in ten hours, his velocity is five miles an hour. 

The velocity of a body is termed relative^ when compared 
with that of another body which is itself in motion. For 
instance, if one man w^alks at the rate of a nnle an hour, and 
another at the rate of two miles an hour, the relative velocity 
of the latter is double that of the former, but the absolute 
velocity of the one is one mile, and that of the other two miles 
an hour. 

Emihj. Let me see if I understand it. The relative ve- 
locity of a body is the degree of rapidity of its motion compared 
\Y\t\i that of another body ; thus if one ship sail three times as 
far as another ship in the same space of time, the velocity of 
the former is equal to three times that of the latter. 

Mrs. B. The general rule may be expressed thus : the 
velocity of a body is measured by the space over w hich it 
moves, divided by the time which it employs in that motion : 
thus if you travel one hundred miles in twenty hours, what is 
your velocity in each hour ? 

Emlbf. I must divide the space, which is one hundred 
miles, by the time, which is twenty hours, and the answer w ill 
be five miles an hour. Then, Mrs. B., mav we not reverse 



40 ON THE LAWS OF MOTION. 

this mle and say, that the time is equal to the space divided by 
the velocity ; since the space one hundred miles, divided by 
the velocity five miles, gives twenty hours for the time ? 

Mrs. B, Certainly ; and we may say also that space is 
equal to the velocity multiplied by the time. Can you tell 
me, Caroline, how many miles you will have travelled, if your 
velocity is three miles an hour and 3 ou travel six hours ? 

Caroline, Eighteen miles ; for the product of 3 multiplied 
by 6, is 18. 

Mrs. B. I suppose that you understand what is meant by 
the terms uniform^ accelerated and retarded motion. 

Emili/. I conceive uniform motion to be that of a body 
whose motion is regular, and at an equal rate throughout ; 
for instance, a horse that goes an equal number of miles every 
hour. But the hand of a w^atch is a much better example, as 
its motion is so regular as to indicate the time. 

M7^s. B. You have a right idea of uniform motion ; but it 
would be more correctly expressed by saying, that the motion 
of a body is uniform when it passes over equal spaces in equal 
times. Uniform motion is produced by a force having acted 
on a body once, and having ceased to act ; as for instance^ 
the stroke of a bat on a cricket ball. 

Caroline. But the motion of a cricket ball is not uniform ; 
its velocity gradually diminishes till it falls to the ground. 

Mrs. B. Recollect that the cricket ball is inert, and has 
no more power to stop than to put itself in motion ; if it falls, 
therefore, it must be stopped by some force superior to that 
hy wlTich it was projected, and v/hich destroys its motion. 

Caroline. And it is no doubt the force of gravity which 
3ui:teracts and destroys that of projection ; but if there were 

) such power as gravity, would the cricket ball never stop ? 

Mrs. B. If neither gi-avity nor any .other force, such a:> 
ilie resistance of the air, opposed its motion, the cricket ball, 
or even a stone thrown by the hand, would proceed onwards 
in a right line, and with an uniform velocity for ever. 

Caroline. You astonish me ! I thought that it was impos- 
sible to produce perpetual motion ? 

Mrs. B. Perpetual motion cannot be produced by art, 
because gravity ultimately destroys all motion that human 
powers can produce. 

Emily. But independently of tlie force of gravity, I can- 
not conceive that the little motion I am capable of giving to 
a stone would put it in motion for ev^r. 



ON THE LAWS OF MOTIOIS. 41 

Mrs, B, The quantity of motion you communicate to the 
stone would not influence its duration ; if you threw it with 
httle force it would move slowly, for its velocity, you must 
remember, will be proportional to the force with which it is 
projected ; but if there is nothing to obstruct its passage, it 
will continue to move with the same velocity, and in the same 
direction as when you first projected it. 

Caroline. This appears to me quite incomprehensible ; 
we do not meet with a single instance of it in nature. 

Mrs, B, I beg your pardon. When you come to study 
the motion of the celestial bodies, you will find that nature 
abounds with examples of perpetual motion ; and that it 
conduces as much to the harmony of the system of the universe, 
as the prevalence of it would to the destruction of all comfort 
on our globe. The wisdom of Providence has therefore 
ordained insurmountable obstacles to perpetual motion here 
below, and though these obstacles often compel us to contend 
with great difficulties, yet there results from it that order, 
regularity and repose, so essential to the preservation of all 
the various beings of which this world is composed. 

Now can you tell me what is retarded motion ? 

Caroline, Retarded motion is that of a body which moves 
every moment slower and slower : thus when I am tired with 
walking fast, I slacken my pace ; or when a stone is thrown 
upwards, its velocity is gradually diminished by the power of 
gravity. 

Mrs, B, Retarded motion is produced by some force act- 
ing upon the body in a direction opposite to that which first 
put it in motion : you who are an animated being, endowed 
with power and will, may slacken your pace, or stop to rest 
when you are tired ; but inert matter is incapable of any feel- 
ing of fatigue, can never slacken its pace, and never stop, 
unless retarded or arrested in its course by some opposing 
force ; and as it is the laws of inert bodies which mechanics 
treats of, I prefer your illustration of the stone retarded in its 
ascent. Now, Emily, it is your turn ; what is accelerated 
motion ? 

Emili/, Accelerated motion, I suppose, takes place when 
the velocity of a body is increased ; if you had not objected 
to our giving such active bodies as ourselves as examples, I 
should say that my motion is accelerated if I change my pace 
from walking to running. I cannot think of any instance of 
4* 



42 ON THE LAWS OF MOTION. 

accelerated motion in inanimate bodies ; all motion of inert 
matter seems to be retarded by gravity. 

Mrs, B. Not in all cases ; for the power of gravitation 
sometimes produces accelerated motion ; for instance, a stone 
falling from a height moves with a regularly accelerated motion. 

Emihj. True ; because the nearer it approaches the earth, 
the more it is attracted by it. 

Mrs, B, You have mistaken the cause of its acceleration 
of motion ; for though it is true that the force of gravity 
increases as a body approaches the earth, the difference is so 
trifling at any small distance from its surface as not to be 
perceptible. 

Accelerated motion is produced when the force which put 
a body in motion continues to act upon it during its motion, 
so that its motion is continually increased. When a stone 
falls from a lieight, the impulse which it receives from gravity 
during the first instant of its fall, would be sufficient to bring it 
to the ground with a uniform velocity : for, as we have ob- 
served, a body having been once acted upon by a force, will 
continue to move w^itli a uniform velocity ; but the stone is 
not acted upon by gravity merely at the first instant of its fall, 
this power continues to impel it during the whole of its descent, 
and it is this continued impulse which accelerates its motion. 

Emily, I do not quite understand that. 

Mrs, B. Let us suppose that the instant after you have let 
fall a stone from a high tower, the force of gravity were anni- 
liilated, the body would nevertheless continue to move down- 
wards, for it v/ould have received a first impulse from gravity, 
and a body once put in motion will not stop unless it meets 
with some obstacle to impede its course ; in this case its 
velocity would be uniform, for though there w^ould be no 
obstacle to obstruct its descent, there would be no force to 
accelerate it. 

Emily, That is very clear. 

Mrs, B, Then you have only to add the power of gravity 
constantly acting on the stone during its descent, and it will 
not be difficult to understand that its motion will become 
accelerated, since the gravity which acts on the stone during 
the first instant of its descent, will continue in force every 
instant till it reaches the ground. Let us suppose that the 
impulse given by gravity to the stone during the first instant 
of its descent be equal to one^ the next instant we shall find 



ON THE LAWS OF MOTION. 46 

that an additional impulse gives the stone an additional velo- 
city equal to one, so that the accumulated velocity is now 
equal to two ; the following instant another im.pulse increases 
the velocity to three, and so on till the stone reaches the 
ground. 

Caroline. Now I understand it ; the effects of preceding 
impulses must be added to the subsequent velocities. 

Blrs, B, Yes ; it has been ascertained both by experiment 
and calculations, which it would be too difficult for us to enter 
into, that heavj/ bodies descending from a height by the force 
of gravity, fall sixteen feet the first second of time, three times 
that distance in the next, five times in the third second, seven 
times in the fourth, and so on, regularly increasing their velo- 
cities according to the number of seconds during which tlie 
body has been falling. 

Emily. If you throw a stone perpendicularly upwards, is 
it not the same length of time ascending that it is descending. 

Mrs. B. Exactly ; in ascending, the velocity is diminished 
by the force of gravity ; in descending, it is accelerated by it. 

Caroline. I should then have imagined that it would have 
fallen quicker than it rose ? 

Mrs. B. You must recollect that the force with v>"hich it is 
projected must be taken into the account ; and that this force 
is overcome and destroyed by gravity before the body falls. 

Caroline. But the force of projection given to a stone in 
throwing it -upwards, cannot alwajs be equal to the force of 
gravity in bringing it down again, for the force of gravity is 
always the same, whilst the degree of impulse given to the 
stone is optional ; I may throw it up gently or v/itli violence. 

Mrs. B. If you throw it gently, it will not rise high ; per- 
haps only sixteen feet, in which case it will fall in one second 
of tiine. Now it is proved by experiment, tha,t an impulse 
requisite to project a body sixteen feet upwards, will make it 
ascend that height in one second ; here then the times of the 
ascent and descent are equal. But supposing it be required 
to throw a stone twice that height, the force must •be propor- 
tionally greater. 

You see then, that the impulse of projection in throwing a 
body upwards, is always equal to the action of the force ot 
gravity during its descent ; and that it is the greater or less 
distance to which the body rises, that makes these two forces 
balance each other. 

I must now explain to you what is meant by the momeninm 



44 ON THE LAWS OF MOTION. 

of bodies. It is the force, or power, with which a body in 
motion, strikes against another body. The momentum of a 
bod}^ is composed of its quantity of matter^ muhiphed by its 
quantity of motion ; in other words, its weight and its velocity. 

Caroline, The quicker a body moves, the greater, no 
doubt, must be the force with which it would strike aeainst 
another body. 

Emily. Therefore a small body may have a greater mo- 
mentum than a large one, provided its velocity be sufficiently 
greater ; for instance, the momentum ol' an arrow shot from 
a bow, must be greater than a stone throvv^n by the hand. 

Caroline, We know also by experience, that the heavier 
a body is, the greater is its force ; it is not therefore difficult 
to understand, that the whole power or momentum of a body 
must be composed of these two properties : but I do not under- 
stand, why they should be muUipUed^ the one by the other ; 
I should have supposed fTiat the quantity of matter should 
have been added to the quantity of motion ? 

Mrs, B, It is found by experiment, tliat if the weight of a 
body is represented by the number 3, and its velocity also by 
3, its momentum will be represented by 9 ; not 6, as would 
be the case, were these figures added, instead of being multi- 
plied together. I recommend it to you to be careful to remem- 
ber the definition of the momentum of bodies, as it is one of 
the most important points in mechanics ; you will find, that 
it is from opposing motion to matter, that machines derive 
their powers.* 

The re-action of bodies, is the next law of motion which I 
must explain to you. When a body in motion strikes against 
another body, it meets with resistance from it ; the resistance 
of the bod}^ at rest, will be equal to the blow struck by the 
body in motion ; or to express myself in philosophical language,. 
action and re-action will be equal, and in opposite directions. 

Caroline, Do you mean to say, that the action of the 
body which strikes, is returned with equal force by the body 
which receives the blow, 

* Tn comparing together the momenta of different bodies, we must be 
attentive to measure their Meighis and velocities, by the same denomina- 
tion of weights and of spaces, otherwise the results woald not agree. 
Thus if we estimate tlie weight of OJie body in on? ces, we must estimate 
the weight of the rest also in ounces, and not in pounds ; and in compu- 
ti'igthe velocities, in Tike manner, we should adhere to the same standai*(l 
of measure, both of space and of time; as for instance, the number of 
feet in one second, or of miles in one hour. 



ON THE LAWS OF MOTION. 45 

Mrs, B. Exactly. 

Caroline. But if a man strikes another on the face with 
his fistj he surely does not receive as much pain by the re- 
action as he inflicts by the blow ? 

Mrs. B. iVo ; but this is simply owing to the knuckles 
having much less feeling than the face. 

Here are two ivory balls suspended by direads, (plate 1. 
fig. 3.) draw one of them, A, a little on one side, — now let it 
go ; — it strikes, you see, against the other ball B, and drives 
it off, to a distance equal to that through which the first ball 
fell ; but the motion of A is stopped, because when it struck 
B, it received in return a blov/ equal to that it gave, and its 
motion w^as consequently destroyed. 

Emily. I should have supposed that the motion of the ball 
A was destroyed, because it had communicated all its motion 
to B. 

Mrs. B. It is perfectly true, that when one body strikes 
against another, the quantity of motion communicated to the 
second body, is lost by the first ; but this loss proceeds from 
the action of the body which is struck. 

Here are six ivory balls hanging in a row, (fig. 4.) draw 
the first out of the perpendicular, and let it fall against the 
second. None of the balls appear to move, you see, except 
the last, which flies off as far as the first ball fell ; can you 
explain this ? 

Caroline. I believe so. When the first ball struck the 
second, it received a blow in return, w^hich destroyed its mo- 
tion ; the second ball, though it did not appear to move, must 
have struck against the third ; the re-action of which set it at 
rest ; the action of the third ball must have been destroyed 
by the re-action of the fourth, and so on till motion was 
commxunicated to the last ball, which, not being re-acted upon, 
ifies off. 

M/'5. B. Very v/ell explained. Objerve, that it is only 
when bodies are clastic, as these ivory balls are, that the 
stroke returned is equal to the stroke given. I w^ill show you 
!«the difference Vv'ith these tv/o balls of clay, (fig. 5.) which are 
not elastic ; when you raise one of these, D, out of the per- 
pendicular, and let it fall against the other, E, the re-action 
of the Ia,tter, on account of its not being elastic, is not sufficient 
to destroy the motion of the former ; only part of the motion 
of D will be communicated to E, and the tw^o balls will move 



16 



ON THE LAWS OF MOTION. 



on together to d and e, which is not to so great a distance as 
that tlirough whicli D fell. 

Observe how useful re-action is in nature. Birds in flying 
strike the air with their wings, and it is the re-action of the air 
which enables them to rise, or advance forwards ; re-action 
being always in a contrary direction to action. 

Caroline, I thought that birds might be lighter than the 
air, when their wings were expanded, and by that means 
enabled to fly. 

.' rs,B. \Vhen their wings are spread, they are better 
supported by the air, as they cover a greater extent of sur- 
face ; but the}' are still much too heavy to remain in that situ- 
ation, without continually flapping their wings, as you may 
have noticed, w hen biids hover over their nests : the force 
with which their wings strike against the air must equal the 
weight of their bodies, in order that the re-action of the air 
may be able to support that weight ; the bird will then re- 
main stationary. If the stroke of the wings, is greater than 
is required merely to support the bird, the re-action of the air 
will make it rise ; if it be less, it will gently descend ; and 
you may have observed the lark, sometimes remaining with 
its wings extended, but motionless : in this state it drops 
rapidly into its nest. 

Caroline, What a beautiful effect this is of the law of re-ac- 
tion ! But if flying is merely a mechanical operation, Mrs. B.^ 
why should we not construct wings, adapted to the size of our 
bodies, fasten them to our shoulders, move them with our 
arms, and soar into the air. 

Mrs, B, Such an experiment has been repeatedly at- 
tempted, but never with success ; and it is now considered 
?.s totally impracticable. The muscular power of birds is 
greater in proportion to their weight than that of man ; were 
we therefore furnished with wings sufficiently large to enable 
us to fly, we should not have strength to put them in motion. 

In swimming, a similar action is produced on the water, as 
that on the air in flying ; and also in rov/ing ; you strike the 
water with the oars, in a direction opposite to that in tvhich the 
boat is required to move, and it is the re-action of the water 
on the oars which drives the boat along. 

Emily, You said, that it was in ehistlc bodies only, that 
re-action was equal to action ; pray what l^odles are elastic 
besides the air. 



ON THE LAWS OF MOTION. 47 

Mrs, B. In speaking of the air, I think we defined elasti- 
city to be a property, by means of which bodies that are 
compressed returned to their former state. If I bend this 
, cane, as soon as I leave it at liberty it recovers its former 
■ position; if I press my finger upon your arm, as soon as I 
remove it, the flesh, by virtue of its elasticity, rises and des- 
troys the impression I made. Of all bodies, the air is the 
most eminent for this property, and it has thence obtained the 
name of elastic fluid. Hard bodies are in the next degree 
elastic ; if two ivory, or metallic balls are struck together, the 
parts at which they touch will be flattened : but their elasticity 
will make them instantaneously resume their former shape. 

Caroline, But when two ivory balls strike against each 
other, as they constantly do on a biUiard table, no mark or 
impression is made by the stroke. 

Mrs. B. I beg your pardon ; but you cannot perceive 
any mark, because their elasticity instantly destroys all trace 
of It. 

Soft bodies, which easily retain impression, such as clay, 
wax, tallow, butter, &c. have very little elasticity ; but of all 
descriptions of bodies liquids are the least elastic. 

Emily, If sealing-wax were elastic, instead of retaining 
the impression of a seal, it would resume a smooth surface as 
soon as the weight of the seal was removed. But pray what 
is it that produces the elasticity of bodies ? 

Mrs, B. There is great diversity of opinion upon that 
point, and I cannot pretend to decide which approaches 
nearest to the truth. Elasticity implies susceptibility of com- 
pression, and the susceptibility of compression, depends upon 
the porosity of bodies, for were there no pores or spares 
between the particles of matter of which a body is composed, 
it could not be compressed. 

Caroline, That is to say, that if the particles of bodies 
were as close together as possible, they cuuld not be squeezed 
closer. 
j^ Emily, Bodies then, whose particles are most distant 
from each other, must be most susceptible of compression, and 
consequently most elastic ; and this you say is the case with 
air, which is perhaps the least dense of all bodies ? 

Mrs. B, You will not in general find this rule hold good, 
for liquids have scarcely any elasticity, whilst hard bodies are 
eminent for this property, though the latter are certainly of 
much greater density than the former ; elasticity implies, 



48 uiN THE LA^^s of motion. 

therefore, not only a susceptibility of compression, but depends 
upon the power of resumincr its former state after compression. 

Caroline, But surely there can be no pores in ivory and 
metals, Mrs. B. ; how then can they be susceptible of 
compression ? 

Mrs, B, The pores of such bodies are invisible to the 
naked eye, but you must not thence conclude that they have 
none ; it is, on the contrary, well ascertained that gold, one 
of the most dense of all bodies, is extremely porous, and that 
these pores are sufficiently large to admit water when strongly 
compressed to pass through them. This was shown by a 
celebrated experiment made many years ago at Florence. 

Emily, If water can pass through gold, there must cer- 
tainly be pores or interstices which afford it a passage ; and if 
gold is so porous, what must other bodies be, which are so 
much less dense than gold ! 

Mrs, B. The chief difference in this respect is, I believe, 
that the pores in some bodies are larger than in others ; in 
cork, sponge, and bread, they form considerable cavities ; in 
wood and stone, when not polished, they are generally- 
perceptible to the naked eye ; whilst in ivorj-, metals, and all 
varnished and polished bodies, they cannot be discerned. To 
give you an idea of the extreme porosity of bodies, sir Isaac 
Newton conjectured that if the earth were so compressed as 
to be absolutely without pores, its dimensions might possibly 
not be more than a cubic inch. 

Caroline, What an idea ! Were we not indebted to sir 
Isaac Newton for the theory of attraction, I should be tempted 
to laugh at him for such a supposition. What insignitic^nt 
little creatures we should be ! 

Mrs, B, If our consequence arose from the size of our 
bodies we should indeed be but pigmies, but remember that 
the mind of Newton was not circumscribed by the dimensions 
of its envelope. 

Emily, It is, however, fortunate that heat keeps the pores 
of matter open and distended, and prevents the attraction of^ 
cohesion from squeezing us into a nut-shell. 

Mrs. B. Let us now return to the subject of re-action, on 
which we have some further observations to make. It is 
re-action, being contrary to action, which produces reflected 
Qnotion, If you throw a ball against the wall, it rebounds ; 
this return of the ball is owing to the re-action of the wall 
against which it struck, and is called reflected motion. 



I'i^f. 1. 



PLATE 71. 





ON THE LAWS OF MOTION. 49 

Emily. And I now understand why balls filled with air 
rebound better than those stuffed with bran and wool, air being 
most susceptible of compression and most elastic, the re-action 
is more complete. 

Caroline, I have observed that when I throw a ball 
straight against the wall, it returns straight to my hand ; but 
if I throw it obliquely upwards, it rebounds still higher, and I 
catch it when it falls. 

Mrs, B, You should not say straight, but perpendicularly 
against the wall ; for straight, is a general term for lines in 
all directions which are neither curved nor bent, and is there- 
fore equally applicable to oblique or perpendicular lines. 

Caroline, I thought that perpendicularly meant either 
directly upwards or downwards. 

Mrs, B, In those directions lines are perpendicular to the 
earth. A perpendicular line has always a reference to some- 
thing towards which it is perpendicular ; that is to say, that 
it inclines neither to the one side nor the other, but makes an 
equal angle on every side. Do you understand what an 
angle is ? 

Caroline. Yes, I believe so : it is two lines meeting in a 
point. 

Mrs, B, Well then, let the line A B (plate II, fig. 1.) 
1'epresent the floor of the room, and the line C D that in which 
you throw a ball against it ; the line C D you will observe, 
forms two angles with the line A B, and those two angles are 
equal. 

Emily, How can the angles be equal, while the lines 
which compose them are of unequal length ? 

Mrs, B, An angle is not measured by the length of the 
lines, but by their opening. 

Emily, Yet the longer the lines are, the greater is the 
opening between them. 

Mrs, B, Take a pair of compasses and draw a circle over 
these angles, making the angular point the centre. 

Emily, To what extent must I open the compasses ? 

Mrs. K You may draw the circle what size you please, 
provided that it cuts the lines of the angles we are to measure. 
All circles, of whatever dimensions, are supposed to be divided 
into 360 equal parts, called degrees ; the opening of an 
angle, being therefore a portion of a circle, must contain a 
certain number of degrees : the larger the angle, the greater 
5 



UN I UK LAWS OFMuTlO.N. 



the number of degrees, and the two angles are said to be equal 
when they contain an equal number of degrees. 

Emily, Now I understand it. As the dimensions of an 
angle depend upon the number of degrees contained between 
its lines, it is the opening and not the length of its lines, which 
determines the size of the angle. 

^irs, B, Very well : now that you have a clear idea of 
the dimensions of angles, can you tell me how many degrees 
are contained in the two angles formed by one line falling 
perpendicular on another, as in the figure I have just drawn ? 

Emily, You must allow me to put one foot of the com- 
passes at the point of the angles, and draw a circle round 
them, and then I think I shall be able to answer your ques- 
tion : the two angles are together just equal to half a circle, 
they contain therefore 90 degrees each ; 90 degrees being a 
quarter of 360. 

Mrs, B, An rmgle of 90 degrees is called a right angle, 
and w^hen one line is perpendicular to another, it forms, you 
see (fig. 1.) a right angle on either side. Angles containing 
more than 90 degrees are called obtuse angles (fig. 2. ;) and 
those containing less than 90 degrees are called acute angles, 
(fig. 3.) 

Caroline, The angles of this square table are right angles, 
but those of the octagon table are obtuse angles ; and the an- 
gles of sharp-pointed instruments are acute angles. 

Mrs, B, Very well. To return now to your observation, 
that if a ball is thrown obliquely against the wall it will not 
rebound in the same direction ; tell me, have you ever played 
at billiards ? 

Caroline, Yes, frequently ; and I have observed that 
when I push the ball perpendicularly against the cushion it 
returns in the same direction ; but when I send it obliquely 
to the cushion, it rebounds obliquely, but on the opposite 
side ; the ball in this latter case describes an angle, the point 
of which is at the cushion. I have observed too, that the 
more obliquely the ball is struck against the cushion, the 
more obliquely it rebounds on the opposite side, so that a 
billiard player can calculate with great accuracy in what di- 
rection it will return. 

Mrs, B, Very well. This figure (fig. 4. plate II.) rep- 
resents a billiard table ; now if you draw a line A B from the 
point where the ball A strikes perpendicular to the cushion ; 



ox THE LAWS OP MOTION. 51 

vou will find that it will divide the angle v/hich the ball de- 
scribes into two parts, or two angles ; the one will show the 
obliquity of the direction of the ball in its passage tow^ards 
the cushion, the other its obliquity in its passage back from 
the cushion. The first is called the angle of incidence^ the 
other the angle of reflection^ and these angles are always 
equal. 

Caroline. This then is the reason why, when I throw a 
ball obliquely against the wall, it rebounds in an opposite 
oblique direction, forming equal angles of incidence and of 
reflection. 

Mrs. B. Certainly ; and you will find that the more ob- 
liquely you throw the ball, the more obliquely it will rebound. 

We must now conclude : but I shall have some further 
observations to make upon the lav/s of motion, at our next 
meeting. 



CONVERSATION IV. 



ON COMPOUND MOTION. 

Compound Motion^ the Result of two Opposite Forces ; Of 
Circular Motion^ the Result of tivo Forces^ one of lohich 
confines the Body to a Fixed Point ; Centre of Motion^ 
the Point at Rest while the other Parts of the Body move 
round it ; Centre of Magnitude, the Middle of a Body ; 
Centripetal Force ^ that which confines a Body to a fixed 
Central Point ; Centrifugal Force, that which impels a 
Body to fly from the Centre ; Fall of Bodies in a Para- 
bola ; Centre of Gravity, the Centre of Weight, or point 
about which the Parts balance each other. 



MRS. B. 

I MUST now explain to you the nature ol' compound motion. 
Let us suppose a body to be struck by two equal forces in 
opposite directions^ how will it move ? 

Emily, If the directions of the forces are in exact oppo- 
sition to each other, I suppose the body would not move 
at all. 

M?'s, B, You are perfectly right ; but if the forces, instead 
of acting on the body in opposition, strike it in two directions 
inclined to each other, at an angle of ninety degrees, if the 
ball A (fig. 5, plate II.) be struck by equal forces at X and 
at Y, will it not move ? 

Emily, The force X would send it towards B, and the 
force Y towards C, and since these forces are equal, I do not 
know how the body can obey one impulse rather than the 
other, and yet I think the ball would move, because as the 
two forces do not act in direct opposition, they cannot entire- , 
ly destroy the effect of each other 



ON COMPOUND MOTION, 53 

Mrs. B. Very true ; the ball will therefore follow the di- 
rection of neither of the forces, but will move in a line between 
them, and will reach D in the same space of time, that the 
force X v/ouldhave sent it to B, and the force Y would have 
sent it to C. Now if you drav/ two lines from D, to join B 
and C, you will form a square, and the oblique line which the 
body describes is called the diagonal of the square. 

Caroline. That is very clear, but supposing the two forces 
to be unequal, that the force X, for instance, be twice as great 
as the force Y ? 

Mrs. B. Then the force X would drive the ball twice as 
far as the force Y, consequently you must draw the line A B 
{fig. 6.,) twice as long as the line A C, the body will in this 
case move to D ; and if you draw lines from that point to B 
and C, you will find that the ball has moved in the diagonal 
of a rectangle. 

Emily. Allow me to put another case ? Suppose the two 
forces are unequal, but do not act on the ball in the direction 
of a right angle, but in that of an acute angle, what will 
result ? 

Mrs. B. Prolong the lines in the directions of the two 
forces, and \ou will soon discover which way the ball will be 
impelled ; it will move from A to D, in the diagonal of a^ 
parallelogram, (fig. 7.) Forces acting in the direction of lines 
forming an obtuse angle, will also produce motion in the 
diagonal of a parallelogram. For instance, if the body set out 
from B, instead of A, and was impelled by the forces X and 
Y, it would move in the dotted diagonal B C. 

We may now proceed to circular motion : this fs the result 
of two forces on a body, by one of which it is projected for- 
v/ard in a right line, whilst by the other it is confined to a 
fixed point. For instance, when I whirl this ball, which is 
fastened to my hand with a string, the ball moves in a circular 
direction ; because it is acted on by two forces, that which I 
give it which represents the force of projection, and that of 
the string which confines it to niy hand. If during its motion 
you were suddenly to cut the string, the ball would fly off* in 
a straight line ; being released from confinement to the fixed 
point, it would be acted on but by one force, and motion pro- 
duced by one force, you knov/, is always in a right line. 

Caroline. This is a little more difficult to comprehend 
than compound motion in straight lines. 

Mrs. B. You have seen a mop trundled, and have observ- 
5* 



54 ON eo3rpouN» motion^* 

ed, that the threads which compose the head of the mop fly 
from the centre ; but being confined to it at one end, they 
cannot part from it ; whilst the water they contain, being 
unconfined, is thrown off in straight lines. 

Emily. In the same way, the flyers of a windmill, when 
put in motion by the wind, would be driven straight forwards 
in a right line, were they not confined to a fixed point round 
which they are compelled to move. 

Mrs, B, Very well. And observe, that the point to which 
the motion of a small body, such as the ball with the stringy 
which may be considered as revolving in one plane, is confin- 
ed, becomes the centre of its motion. But when the bodies 
are not of a size or shape to allow of our considering every 
part of them as moving in the same plane, they in reality re- 
volve round a line, which line is called the axis of motion. 
In a top, for instojice, when spinning on its point, the axis is 
the line which passes through the middle of it, perpendicular- 
ly to the floor. 

Caroline. Tlie axle of the flyers of the windmil?, is then 
the axis of its motion ; but is the centre of motion always in 
the middle of a body ? 

Mrs. B. No, not always. The middle point of a body, is 
called its centre of magnitude, or position, that is the centre 
of its mass or bulk. Bodies have also another centre, called 
the centre of gravity, which I shall explain to you ; but at 
present we must confine ourselves to the axis of motion. 
This line you must observe remains at rest ; whilst all the 
other parts of the body move around it ; when you spin a top 
the axis is stationary vrhilst every other part is in motion 
round it. 

Caroline. But a top generally has a motion forwards, 
besides its spinning motion ; and then no point within it can 
be at rest ? 

Mrs. B. What I say of the axis of nwtion, relates only to 
circular motion ; that is to say, to motion round a line, and 
not to that which a body may have at the same time in any 
other direction. There is one circumstance in circular mo- 
tion, wliich you must carefully attend to ; which is, that the 
iiirther any part of a body is from the axis of motion, the 
greater is its velocity ; as you approach that line, the velocity 
of the parts gradually diminish till you reach the axis of 
motion, which is perfectly at rest. 

Cwroiine*, But^ if every part of the same body did not 



F^<J• 1. 



PLATE 




Fiq. 6. 



Fia.o- 




^.4. - V I 



%• 7- 





Fi^. O. 



ON COMPOUND MOTION. 55 

move with the same velocity, that part which moved quickest^ 
must be separated from the rest of the body, and leave it 
behind ? 

Mrs. B. You perplex yourself by confounding the idea of 
circular motion, with that of motion in a right line ; you must 
think only of the motion of a body round a fixed line, and you 
will find, that if the parts farthest from the centre had not the 
greatest velocity, those parts would not be able to keep up 
with the rest of the body, and would be left behind. Do not 
the extremities of the vanes of a windmill move over a much 
greater space, than the parts nearest the axis of motion ? 
(pi. III. fig. 1.) the three dotted circles describe the paths in 
which three different parts of the vanes move, and though the 
circles are of different dimensions the vanes describe each of 
them in the same space of time. 

Carolinp, Certainly they do ; and I now only wonder^ 
that we neither of us ever made the observation before : and 
the same effect must take place in a solid body, like the top 
in spinning ; the most bulging part of the surface must move 
with the greatest rapidity. 

Mrs, B. The force which confines a body to a centre, 
round which it moves is called the centripetal force ; and 
that force, which impels a body to fly from the centre, is cal- 
led the centrifugal force ; in circular motion these two forces, 
constantly balance each other ; otherwise the revolving body 
would either approach the centre^ or recede from it, according 
as the one or the other prevailed. 

Caroline. When I see any body moving in a circle, I shall 
remember, that it is acted on by two forces. 

Mrs. B. Motion, either in a circle, an ellipsis, or any 
other curve-line, must be the result of the action of two for- 
ces ; for you know, that the impulse of one single force^ always 
produces motion in a right line. 

Emily. And if any cause should destroy the centripetal 
force, the centrifugal force would alone impel the body, and 
it would I suppose fly off in a s^traight line from the centre to 
which it had been confined. 

Mrs. B. It would not fly off in a right line from the cen- 
tre ; but in a right line in the direction in which it was mo- 
ving, at the instant of its release ; if a stone, whirled- round 
in a sling, gets loose at the point A (plate III. fig, 2.) it flies 
off in the direction A B ; this line is called a tangenty it 
touches the circumference of the circlej and forms a right 



00 ON COMPOUND MOTION. 

angle with a line drawn from that point of the circumference 
to the centre of the circle^ C. 

Emily. You say, that motion in a curve-line, is owing to 
two forces acting upon a body ; but when I throw this ball 
in a horizontal direction^ it describes a curve line in falling ; 
and yet it is only acted upon by the force of projection ; there 
is no centripetal force to confine it^ or produce compound 
motion. 

Mrs, B, A ball thus thrown, is acted upon by no less than 
three forces ; the force of projection, which you communica- 
ted to it ; the resistance of the air thr ough which it passes, 
which diminishes its velocity, without changing its direction ; 
and the force of gravity, which finally brings it to the ground. 
The power of gravity, and the resistance of the air, being 
always greater than any force of projection we can give a 
body, the latter is gradually overcome, and the body brought 
to the ground ; but the stronger the projectile force, the lon- 
ger will these powers be in subduing it, and the further the 
body will go before it falls. 

Caroline. A shot fired from a cannon, for instance, will 
go much further, than a stone projected by the hand. 

Mrs. B. Bodies thus projected, 3^ou observed, described a 
curve-line in their descent ; can you account for that ? 

Caroline. No ; I do not understand, why it should not 
fall in the diagonal of a square. 

Mrs. B. You must consider that the force of projection is 
strongest when the ball is first thrown ; this force, as it pro- 
ceeds, being weakened by the continued resistance of the air, 
the stone, therefore, begins by moving in a horizontal direc- 
tion ; but as the stronger powers prevail, the direction of the 
ball will gradually change from a horizontal, to a perpendicu- 
lar line. Projection alone, would drive the ball A, to B,. 
(fig. 3) gravity would bring it to C ; therefore, when acted 
on in different directions, by these two forces, it moves be- 
tween, gradually inclining more and more to the force of 
gravity, in proportion as this accumulates ; instead therefore 
of reaching the ground at D, as you supposed it would, it falls 
somewhere about E. 

Caroline. It is precisely so ; look, Emily, as 1 throw 
this ball directly upwards, how the resistance of the air and 
gravity conquers projection. Now I will throw it upwards 
obliquely : see the force of projection enables it, for an instant^ 



ox COMPOUND MOTION, ^l 

to act in opposition to that of gravity ; but it is soon brought 
down again. 

Mrs, -S. The curve-line which the ball has described, is 
called in geometry a parabola ; but when the ball is thrown 
perpendicularly upwards, it will descend perpendicularly ; 
because the force of projection, and that of gravity, are in the 
same line of direction. 

We have noticed the centres of magnitude, and of motion ; 
but I have not yet explained to you, what is meant by the 
centre of gravity ; it is that point in a body, about which all 
the parts exactly balance each other ; if therefore, that point 
is supported, the body will not fall. Do you understand 
this ? 

Emily. I think so, if the parts round about this point have 
an equal tendency to fall, they will be in equilibrium, and as 
long as this point is supported, the body cannot fall. 

Mrs, B. Caroline, what would be the effect, were any 
other point of the body alone supported ? 

Caroline. The surrounding parts no longer balancing 
each other, the body, I suppose, would fall on the side at 
which the parts are heaviest. 

Mrs, B, Infallibly ; whenever the centre of gravity is 
unsupported, the body must fall. This sometimes happens 
with an overloaded waggon winding up a steep hill, one side 
of the road being more elevated than the other ; let us suppose 
it to slope as is described in this figure, (plate III. fig. 4,) we 
will say, that the centre of gravity of this loaded waggon is at 
the point A. Nowj^our eye will tell you, that a waggon thus 
situated, will overset ; and the reason is, that the centre of 
gravity A, is not supported ; for if you draw a perpendicular 
line from it to the ground at C, it does not fall under the 
waggon within the wheels, and is therefore not supported by 
them. 

Caroline, I understand that perfectly ; but what is the 
meaning of the other point B ? 

Mrs, B, Let us, in imagination, take off the upper part 
cf the load ; the centre of gravity will then change its situa- 
tion, and descend to B, as that will now be the point about 
which the parts of the less heavily laden waggon will balance 
each other. Will the waggon now be upset ? 

Caroline, No, because a perpendicular line from that 
point falls within the wheels at D, and is supported by them ; 
and when the centre of gravity is supported, the body will not 
fall 



■ 'H ON COMPOUND MOTION. 

Emily, Yet I should not much like to })ass a waggon, lu 
that siuiation ; for, as you see, the point D is but just within 
the left wheel ; if the right wheel was merely raised, by pass- 
ing over a stone, the point D would be thrown on the outside 
of the left wheel, and the waggon would upset. 

Caroline. A waggon, or any carriage whatever, will then 
be most firmly supported, when the centre of gravity falls 
exactly between the wheels ; and that is the case in a level 
road. 

Pray, v/hereabouts is the centre of gra\ity of the human 
body ? 

Mrs, B, Between the hips ; and as long as we stand 
upright, this point is supported by the feet ; if you lean on 
one side, you will find that you no longer stand firm. A 
rope-dancer performs all his feats of agility, by dexterously 
supporting his centre of gravity ; whenever he finds that he 
is in danger of losing his balance, he shifts the heavy pole, 
which he holds in his hands, in order to throw the weight 
towards the side that is deficient ; and thus by changing the 
situation of the centre of gravity, he restores his equilibrium. 

Caroline. When a stick is poised on the tip of the finger, 
is it not by supporting its centre of gravity ? 

Mrs. B. Yes ; and it is because the centre of gravity is 
not supported, that spherical bodies roll down a slope. A 
sphere being perfectly round, can touch the slope but by a 
single point, and that point cannot be perpendicularly under 
the centre of gravity, and therefore cannot be supported, as 
you will perceive by examining this figure, (fig. 5. plate III.) 

Emily. So it appears ; yet I have seen a cyhnder of wood 
roll up a slope ; how is that contrived ? 

Mrs. B. It is done by plugging one side of the cylinder 
with lead, as at B. (fig. 5. plate III.) the body being no long- 
er of an uniform density, the centre of gravity is removed 
from the middle of the body to some point in the lead, as that 
substance is much heavier than wood ; now you may observe 
that in order that the cylinder may roll down the plane, as it 
is here situated, the centre of gravity must rise, which is impos- 
sible ; the centre of gravity must always descend in moving, 
and will descend by the nearest and readiest means, which 
will be by forcing the cylinder up the slope, until the centre 
of gravity is supported, and then it stops. 

Caroline. The centre of gravity, therefore, is not always 
in the middle of a body. 

Mrs, B. No, that point we have called the centre ot 



ON COMPOUND MOTION. 59 

inagnitude ; when the body is of an uniform density the centre 
of gravity is in the same point ; but when one part of the body- 
is composed of heavier materials than another part, the centre 
of gravity being the centre of the weight of the body can no 
longer correspond with the centre of magnitude. Thus you 
see the centre of gravity of this cylinder plugged with lead; 
cannot be in the same spot as the centre of magnitude. 

Emily, Bodies, therefore, consisting but of one kind of 
substance, as wood, stone, or lead, and whose densities are 
consequently uniform, must stand more firmly, and be more 
difficult to overset, than bodies composed of a variety of sub- 
stances, of different densities, which ma^ throw the centre of 
gravity on one side. 

Mrs, B, Yes ; but there is another circumstance which 
more materially affects the firmness of their position, and that 
is their form. Bodies that have a narrow base are easily- 
upset, for if they are the least inclined, their centre is no 
longer supported, as you may perceive in fig. 6. 

Caroline, I have often observed with what difficulty a 
person carries a single pail of water ; it is owing, I suppose, 
to the centre of gravity being thrown on one side, and the 
opposite arm is stretched out to endeavor to bring it back, to 
its original situation ; but a pail hanging on each arm is car- 
ried without difficulty, because they balance each other, and 
:the centre of gravity remains supported by the feet. 

Mrs, B, Very well ; I have but one more remark to 
-make on the centre of gravity, v/hich is, that when two bodies 
are fastened together, by a line, string, chain, or any power 
whatever, they are to be considered as forming but one body ; 
if the two bodies be of equal weight, the centre of gravity will 
be in the middle of the line which unites them, (fig. 7) but if 
one be heavier than the other, the centre of gravity will be 
proportionally nearer the heavy body than the light one. 
(fig. 8.) If you were to carry a rod or pole with an equal 
weight fastened at each end of it, you would hold it in the 
middle of the rod, in order that the weights should balance 
each other ; whilst if it had unequal weights at each end you 
would hold it nearest the greater weight, to make them balance 
each other, 

Emily, And in both cases we should sup])ort the centre 
of gravity ; and if one weight be very considerably larger 
than the other, the centre of gravity will be thrown out of the 
rod into the heaviest weight, (fig. 9.) 

Mrs. B, Undoubtedly. 



CONVERSATION V. 



ON THE MECHANICAL POWERS. 

Of the power of Machines ; Of the Lever in General ; Of 
the Lever of the First Kind, having the Fulcrum between 
the Power and the Weight ; Of the Lever of the Second 
Kind, having the Weight between the Po2ver and the 
Fidcrum ; Of the Lever of the Third Kind^ having the 
Power beticeen the Fulcrum and the Weight, 



MRS.B. 

We may now proceed to examine the mechanical powers ; 
they are six in number, one or more of which enters into the 
composition of every machine. The lever, the pulley, the 
wheel and axle, the inclined plane, the wedge, and the screw. 

In order to understand the power of a machine, there are 
four things to be considered. 1st. The power that acts : 
this consists in the effort of men or horses, of weights, springs, 
steam, &;c. 

2dly. The resistance which is to be overcome by the pow- 
er ; this is generally a weight to be moved. The power must 
always be superior to the resistance, otherwise the machine 
could not be put in motion. 

Caroline, If for instance the resistance of a carriage was 
greater than the strength of the horses employed to draw it, 
they would not be able to make it move. 

Mrs. B. 3dly. We are to consider the centre of motion, 
or as it is termed in mechanics, the fulcrum ; this you may- 
recollect is the point about which all the parts of the body- 
move ; and lastly, the respective velocities of the power, and 
of the resistance. 

Emily. That must depend upon their respective distan- 



Tiq. 1. 



PL ATI: IV 





MT 



ON THE RIECHANICAL POWERS, 6i 

t:es from the axis of motion ; as we observed in the motion of 
the vanes of the windmill. 

MiS. B, We shall now examine the power of the lever. 
The lever is an inflexible rod or beam of any kind, that is to 
say, one which will not bend in any direction. For instance, 
the steel rod to which these scales are suspended is a lever, 
and the point in which it is supported the fulcrum, or centre 
of motion ; now, can you tell me why the two scales are in 
equilibrium ? 

Caroline, Being both empty, and of the same weight, 
they balance each other. 

Emily. Or, more correctly speaking, because the centre 
of gravity common to both is supported. 

Mrs, B. Very v/ell ; and which is the centre of gravity of 
this pair of scales ? (fig. 1. plate IV.) 

Emily, You have told us that when two bodies of equal 
weight were fastened together, the centre of gravity was in the 
middle of the line that connected them ; the centre of gravity 
of the scales must therefore be in the fulcrum F of the lever 
which unites the two scales ; and corresponds with the centre 
of motion. 

Caroline. But if the scales contained different weights, 
the centre of gravity would no longer be in the fulcrum of the 
lever, but removed towards that scale which contained the 
heaviest weight ; and since that point would no longer be 
supported, the heav}^ scale would descend and out-weigh the 
other. 

Mrs. B. True ; but tell me, can you imagine any mode 
by which bodies of different weights can be made to balance 
each other, either in a pair of scales, or simply suspended to 
the extremities of the lever ? for the scales are not an essen- 
tial part of the machine, they have no mechanical power, 
and are used merely for the convenience of containing the 
substance to be weighed. 

Caroline. What ! make a light body balance a heavy 
one ? I cannot conceive that possible. 

Mrs. B. The fulcrum of this pair of scales (fig. 2.) is 
moveable, you see ; I can take it off the prop, and fasten it 
on again in another part ; this part is now become the ful- 
crum, but it is no longer in the centre of the lever. 

Caroline. And the scales are no longer true ; for that 
which hangs on the longest side of the lever descends. 
6 



62 ON THE MECHANICAL POWEHS. 

Mrs, B, The two parts of the lever divided by the fulcrum 
are called its arms, you should therefore say the longest arm, 
not the longest side of the lever. These arms are likewise 
frequently distinguished by the appellations of the acting and 
the resisting part of the lever. 

Your observation is true that the balance is now destroy- 
ed ; but it will answer the purpose of enabling you to com- 
prehend the power of a lever when the fulcrum is not in the 
centre. 

Emily. This would be an excellent contrivance for those 
who cheat in the weight of their goods ; by making the ful- 
crum a little on one side, and placing the goods in the scale 
which is suspended to the longest arm of the lever, they 
would appear to weigh more than they do in reality. 

Mrs, B, You do not consider how easily the fraud would 
be detected ; for on the scales being emptied they would not 
hang in equilibrium. 

Emily. True ; I did not think of that circumstance. 
But I do not understand why the longest arm of the lever 
should not be in equilibrium with the other ? 

Caroline. It is because it is heavier than the shortest arm ; 
the centre of gravity, therefore, is no longer supported. 

Mrs. B. You are right ; the fulcrum is no longer in the 
centre of gravity ; but if we can contrive to make the fulcrum 
in its present situation become the centre of gravity, the 
scales will again balance each other ; for you recollect that 
the centre of gravity is that point about which every part of 
the body is in equilibrium. 

Emily. It has just occurred to me how this may be 
accomplished ; put a great weight into the scale suspended 
to the shortest arm of the lever, and a smaller one into that 
suspended to the longest arm. Yes, I have discovered it — 
look, Mrs. B., the scale on the shortest arm will carry Slbs., 
and that on the longest arm only one, to restore the balance. 

Mrs. B. You see, therefore, that it is not so impracticable 
as you imagined to make a heavy body balance a light one ; 
and this is in fact the means by which you thought an impo- 
sition in the weight of goods might be effected, as a weight of 
ten or twelve ounces might thus be made to balance a pound 
of goods. Let us now take off the scales that we may 
consider the lever simply ; and in this state you see that the 



ON THE i^rECHANICAL POWERS. 6S 

(ulcriim is no longer the centre of gravity ; but it is, and 
must ever be^ the centre of motion, as it is the only point 
which remains at rest, while the otlier parts move about it. 

Caroline. It now resembles the two opposite vanes of 
a windmill, and the fulcrum the point round which they 
move. 

Mrs. B. In describing the motion of those vanes, you 
may recollect our observing that the farther a body is from 
the axis of motion, the greater is its velocity. 

Caroline, That I remember and understood perfectly. 

Mrs. B. You comprehend then, that the extremity of 
the longest arm of a lever must move with greater velocity 
than that of the shortest arm ? 

Emili/, No doubt, because it is farthest from the centre of 
motion. And pray, Mrs. B., when my brothers play at 
see-saw, is not the plank on which they ride a kind of lever ? 

Mrs. B. Certainly ; the log of wood v/hich supports it 
from the ground is the fulcrum, and those who ride represent 
the power and the resistance at each end of the lever. And 
have you not observed that when those who ride are of equal 
weight, the plank m^ust be supported in the middle to make 
the two arms equal ; whilst if the persons differ in weight, 
the plank must be drawn a little further over the prop, to 
make the arms unequal, and the lightest person who repre- 
sents the resistance, must be placed at the extremity of the 
longest arm. 

Caroline. That is always the case when I ride on a plank 
with my youngest brother ; I have observed also that the 
lightest person has the best ride, as he moves both further and 
quicker ; and I now understand that it is because he is more 
distant from the centre of motion. 

Mrs. B. The greater velocity with which your little 
brother moves, renders his momentum equal to yours. 

Caroline. Yes ; I have i\ie most gravity, he the greatest 
velocity ; so that upon the whole our momentums are equal. 
But you said, Mrs. B., that the power should be greater than 
the resistance to put the machine in motion ; how then can 
the plank move if the momentums of the persons who ride 
are equal. 

Mrs. B. Because each person at his descent touches the 
ground with his feet ; the reaction of which gives him an 
impulse which increases his velocity ; this spring is requisite 
to destroy the equilibrium of the power and the resistance^ 



M ON TKE MECHANICAL POWERS. 

Otherwise^ the plank would not move. Did you ever observe 
that a lever describes the arc of a circle in its motion ? 

Emily. No ; it appears to me to rise and descend per- 
pendicularly ; at least I always thought so. 

Mrs, B. I believe I must make a sketch of you and your 
brother riding on a plank; in order to convince you of your 
error, (fig. 4. pi. IV.) You may now observe that a lever 
can move only round the fulcrum, since that is the centre of 
motion ; it would be impossible for you to rise perpendicu- 
larly to the point A, or for your brother to descend in a 
straight line to the point B ; you must in rising and he in 
descending describe arcs of your respective circles. This 
drawing shows you also how much superior his velocity 
must be to yours ; for if you could swing quite round, you 
would each complete your respective circles in the same time. 
Caroline, My brother's circle being much the largest he 
must undoubtedly move the quickest. 

Mrs, B, Now tell me, do you think that your brother 
could raise you as easily without the aid of a lever ? 
Caroline, Oh no, he could not lift me off the ground. 
Mrs, B, Then I think you require no further proof of the 
power of a lever, since you see what it enables your brother 
to perform. 

Caroline. I now understand what you meant by saying, 
tliat in mechanics, motion was opposed to matter, for it is my 
brother's velocity which overcomes my weight. 

M)^s, B, You may easily imagine, what enormous weights 
may be raised by levers of this description, for the longer the 
acting part of the lever in comparison to the resisting part, 
the greater is the effect ptoduced by it ; because the greater 
is the velocity of the power compared to that of the weight. 

There are three clifierent kinds of levers ; in the first the 
fulcrum is between the power and the weight. 

Caroline, This kind then com[>rehend3 the several levers 
you have described. 

Mrs, B, Yes, when in levers of tlie first kind, tlie fulcrum 
^ is equally between the power and the weight, as in the bal- 
ance, the power must be greater than the weight, in order to 
move it ; for nothing can in this case be gained by velocity ; 
the two arms of the lever being equal, the velocity of their 
extremities must be so likewise. The balance is therefore of 
no assistance as a mechanical powder, but it is extremely useful 
to estimate the respective v>^eights of bodies. 



ON THE MECHANICAL POWERS. 65 

But when (fig. 5.) the fulcrum F of a lever is not equally 
distant from the power and the weight, and that the power P 
acts at the extremity of the longest arm, it may be less than 
the weight W, its deficiency being compensated by its supe- 
rior velocity ; as we observed in the see-smv, 

Emily. Then when we want to lift a great weight, wt 
must fasten it to the shortest arm of a lever, and apply our 
strength to the longest arm ? 

Mrs, jB. If the case will admit of your putting the end of 
the lever under the weight, no fastening will be required ; as 
you will perceive by stirring the fire. 

Emily, Oh yes ! the poker is a lever of the first kind, 
the point where it rests against the bars of the grate whilst I 
am stirring the fire, is the fulcrum ; the short arm or resisting 
part of the lever is employed in lifting the weight, which is 
the coals, and my hand is the power applied to the longest 
arm, or acting part of the lever. 

Mrs, B, Let me hear, Caroline, whether you can equally 
well explain this instrument, which is composed of two levers, 
united in one common fulcrum. 

Caroline, A pair of scissors ! 

Mrs, B, You are surprised, but if you examine their 
construction, you will discover that it is the power of the lever 
that assists us in cutting with scissors. 

Caroline, Yes ; I now perceive that the point at which 
the two levers are screwed together, is the fulcrum ; the 
handles, to which the power of the fingers is applied, are the 
extremities of the acting part of the levers, and the cutting 
part of the scissors, are the resisting parts of the levers : 
therefore, the longer the handles and the shorter the points of 
the scissors, the more easily you cut with them. 

Emily, That I have often observed, for when I cut 
pasteboard or any hard substance, I always make use of that 
part of the scissors nearest the screw or rivet, and I now 
understand why it increases the power of cutting ; but I con- 
fess that I never should have discovered scissors to have been 
double levers ; and pray are not snufiers levers of a similar 
description ? 

Mrs, B, Yes, and most kinds of pincers ; the great power 
of which consists in the resisting part of the lever being very 
short in comparison of the acting part. 

Caroline,. And of what nature are the two other kinds of 
levers ? 

6* 



66 ON THE MECHANICAL POWERS. 

Mrs. B. In levers of the second kind, the weight, instead 
of being at one end, is situated between the power and the . 
fulcrum, (fig. 6.) 

Caroline, The weight and the fulcrum have here chang- 
ed places ; and what advantage is gained by this kind of 
lever ? 

Mrs, B. In moving it, the velocity of the power must 
necessarily be greater than that of the weight, as it is more 
distant from the centre of the motion. Have you ever seen 
your brother move a snow-ball by means of a strong stick, 
when it became too heavy for him to move without assistance ? 
Caroline, Oh yes ; and this was a lever of the second 
order (fig. 7-) ; the end of the stick, which he thrusts under 
the ball, and which rests on the ground, becomes the fulcrum ; 
the ball is the weight to be moved, and the power his hands 
applied to the other end of the lever. In this instance there 
is an immense difference in the length of the arms of the 
lever ; for the weight is almost close to the fulcrum. 

Mrs, B, And the advantage gained is proportional to 
this difference. Fishermen's boats are by levers of this 
description raised from the ground to be launched into the 
sea, by means of slippery pieces of board which are thrust 
under the keel. The most common example that we have 
of levers of the second kind is in the doors of our apartments. 
EniiJij, The hinges represent the fulcrum, our hands the 
power applied to the other end of the lever ; but where is 
the weight to be moved ? 

Mrs, B, The door is the weight, and it consequently 
occupies the whole of the space between the power and the 
fulcrum. Nutcrackers are double levers of this kind ; the 
hinge is the fulcrum, the nut the resistance, and the hands the 
power. 

In levers of the third kind (fig. 8,), the fulcrum is again at 
one of the extremities, the weight or resistance at the other, 
and it is now the power which is applied between the fulcrum 
and the resistance. 

Emily, The fulcrum, the weight, and the power, then, 
each in their turn, occupy some part of the middle of the lever 
between its extremities. But in this third kind of lever, the 
weight being farther from the centre of motion than the pow- 
er, the difficulty of raising it seems increased rather than 
diminished. 

Mrs, B. That is very true ; a lever of this kind is there- 



ox THE .MECHANICAL POWERS. 6/ 

fore never used, unless absolutely necessary, as is the case in 
lifting up a ladder perpendicularly in order to place it against 
the wall ; the man who raises it cannot place his hands on 
the upper part of the ladder, the power, therefore, is necessa- 
rily placed much nearer the fulcrum than the weight. 

Caroline. Yes, the hands are the power, the ground the 
fulcrum, and the upper part of the ladder the weight. 

Mrs. B. Nature employs this kind of lever in the struc- 
ture of the human frame. In lifting a weight with the hand, 
the lower part of the arm becomes a lever of the third kind ; 
the elbow^ is the fulcrum, the muscles of the fleshy part of 
the arm the power ; and as these are nearer to the elbow than 
the hand, it is necessary that their power should exceed the 
weight to be raised. 

Emily. Is it not surprising that nature should have fur- 
nished us with such disadvantageous levers ? 

Mrs. B. The disadvantage, in respect to power, is more 
than counterbalanced by the convenience resulting from this 
structure of the arm ^ and it is no doubt that which is best 
adapted to enable it to perform its various functions. 

We have dwelt so long on the lever, that we must reserve 
the examination of the other mechanical powers to our next 
interview. 



CONVERSATION V- 

CONTINUED. 



ON THE MECHANICAL POWERS. 

Of the Fitlley ; Of the Wheel and Axle ; Of the Inclined^ 
Plane: OftheJFedge; Of the Screw, 



MRS. B. 

The pulley is the second mechanical power we are to 
examine. You, both, I suppose, have seen a pulley ? 

Caroline. Yes, frequently : it is a circular and flat piece 
of wood or metal, with a string which runs in a groove round 
it ; by means of which, a weight may be pulled up ; thus 
pulleys are used for drawing up curtains. 

M7'S, B. Yes ; but in that instance the pulleys are fixed, 
and do not increase the power to raise the w^eights, as you 
will perceive by this figure, (pi. Y. fig. 1.) Observe that the 
fixed pulley is on the same principle as the lever of a pair of 
scales, in which the fulcrum F being in the centre of gravity, 
the power P and the weight W, are equally distant from it, 
and no advantage is gained. 

Emily, Certainly ; if P represents the power employed 
to raise the weight W, the power must be greater than the 
weight in order to move it. But of what use then are pulleys 
n mechanics ? 

Mrs, B, The next figure represents a pulley which is 
not fixed, (fig. 2.) and thus situated you will perceive that it 
affords us mechanical assistance. In order to raise the weight 
(W) one inch, P, the power, must draw the strings B and C 
one inch each ; the whole string is therefore shortened two 
inches, while the weight is raised only one. 



4 



^^' 



PLATE V. 



F 



■6 -6 m 




©N THE MECHANICAL POWERS. 60 

Emily. That I understand : if P drew the string but one 
inch, the weight would be raised only half an inch, because it 
would shorten the strings B and C half an inch each, and 
consequently the pulley with the weight attached to it, can be 
raised only half an inch. 

Caroline, I am ashamed of my stupidity ; but I confess 
that I do not understand this ; it appears to me that the 
weight would be raised as much as the string is shortened by 
the power. 

Mrs. B. I will endeavour to explain it more clearly. I 
fasten this string to a chair and draw it towards me ; I have 
now shortened the string, by the act of drawing it, one yard. 

Caroline. And the chair, as I supposed, has advanced 
one yard. 

Mrs. B. This exemplifies the nature of a single fixed 
pulley only. Now unfasten the string, and replace the chair 
where it stood before. In order to represent the moveable 
pulley, we must draw the chair forwards by putting the string 
round it ; one end of the string may be fastened to the leg of 
the table, and I shall draw the chair by the other end of the 
string. I have again shortened the string one yard ; how 
much has the chair advanced ? 

Caroline. I now understand it ; the chair represents the 
weight to which the moveable pulley is attached ; and it is 
very clear that the weight can be drawn only half the length 
you draw the string. I believe the circumstance that per- 
plexed me was, that I did not observe the difference that 
resuhs from the weight being attached to the pulley, instead 
of being fastened to the string, as is the case in the fixed pulley. 

Emily. But I do not 3^et understand the advantage of 
pulleys ; they seem to me to increase rather than diminish 
the difficulty of raising weights, since you must draw the 
string double the length that you raise the weight ; whilst 
with a single pulley, or without any pulley, the weight is 
raised as much as the string is shortened. 

Mrs. B. The advantage of a moveable pidley consists in 
dividing the difficulty ; we must draw, it is true, twice the 
length of the string, but then only half the strength is required 
tliat would be necessary to raise the weight without the 
assistance of a moveable pulley. 

Emily. So that the difficulty is overcome in the same 
manner as it would be, by dividing the weight into two equal 
parts, and raising them successively. 



70 ON THE MECHANICAL POWERS. 

Mrs. B, Exactly. You must observe, that with a move- 
able pulley the velocity of the power is double that of the 
weight, since the power P (fig, 2.) moves two inches, whilst 
the weight W moves one inch ; therefore the power need not 
be more than half the weight to make their momentums equal. 

Caroline, Pulleys act then on the same principle as the 
lever, the deficiency of strength of the power being compen- 
sated by its superior velocity. 

Mr5. B. You will find, that all mechanical power is 
founded on the same principle. 

Emily. But may it not be objected to pulleys, that a 
longer time is required to raise a weight by their aid than 
without it ; for what you gain in powder you lose in time ? 

Mrs. B. That, my dear, is the fundamental law in me- 
chanics : it is the case with the lever as well as the pulley ; 
and you will find it to be so with all the other mechanical 
powers. 

Caroline. I do not see any advantage in the mechanical 
powers then, if what we gain by them one way is lost another. 

Mrs. B. Since we are not able to increase our natural 
strength, is not that science of wonderful utility, by means of 
whiv'^h we may reduce the resistance or weight of any body to 
the level of our strength ? This the mechanical powers 
enable us to accomplish, by dividing the resistance of a body 
into parts which we can successively overcome. It is true, 
as you observe, that it requires a sacrifice of time to attain this 
end, but you mast be sensible how very advantageously it is 
exchanged for power : the utmost exertion we can make adds 
but little to our natural strength, whilst we have a much more 
unlimited command of time. You can now understand, that 
the greater the number of pulleys connected by a string, the 
more easily the weight is raised, as the difficulty is divided 
among the number of strings, or rather of parts into which the 
string is divided by the pulleys. Several pulleys thus connec- 
ted, form what is called a system, or tackle of pulleys, (fig. 
3.) You may have seen them suspended from cranes to 
raise goods into warehouses, and in ships to draw up the 
sails. 

Emily. But since a fixed pulley affords us no mechanical 
aid, why is it ever used ? 

Mrs. B. Though it does not increase our power, it is 
frequently useful for altering its direction. A single pulley 
enables us to draw up a curtain, by drawing down the string 



ON THE MECHANICAL POWERS. ?! 

connected with it ; and we should be much at a loss to accom- 
plish this simple operation without its assistance. 

Ca7'oline, There would certainly be some diificulty in 
ascending to the head of the curtain, in order to draw it up. 
Indeed. I now recollect having seen workmen raise small 
w^eights by this means, whicli seemed to answer a very useful 
purpose. 

Mrs. B. In shipping, both the advantages of an increase 
of power and a change of direction, by means of pulleys, are 
united : for the sails are raised up the masts by the sailors on 
deck, from the change of direction wdiich the pulley effects, 
and the labour is facilitated by the mechanical power of a 
combination of pulleys. 

Emily, But the pulleys on ship-board do not appear to 
me to be united in the manner you have shown us. 

Mrs. B. They are, I believe, generally connected as des- 
cribed in figure 4, both for nautical, and a variety of other 
purposes ; but in whatever manner pulleys are connected by 
a single string, the mechanical power is the same. 

The third mechanical powder is the wheel and axle. Let 
us suppose (plate V. fig. 5.) the weight W to be a bucket 
of water in a w ell, w^hich we raise by winding the rope, to 
which it is attached, round the axle ; if this be done without 
a wheel to turn the axle, no mechanical assistance is received. 
The axle without a wheel is as impotent as a single fixed 
pulley, or a lever, whose fulcrum is in the centre ; but add 
the wheel to the axle, and you will immediately find the 
bucket is raised with much less difficulty. The velocity of ^ 
the circumference of the wheel is as much greater than that of 
the axle, as it is further from the centre of motion : for the 
wheel describes a great circle in the same space of time that 
the axle describes a small one, therefore the powder is increas- 
ed in the same proportion as the circumference of the wheel 
is greater than that of the axle. If the velocity of the w heel 
is twelve times greater than that of the axle, a pow er nearly 
twelve times less than the weight of the bucket would be able 
to raise it. 

Emily. The axle acts the part of the shorter arm ol the 
lever, the wheel that of the longer arm. 

Caroline. In raising water there is commonly, I believe^ 
instead of a wheel attached to the axle, only a crooked handle, 
which answers the purpose of winding the rope round the 
axle, and thus raising the bucket. 



72 eS THE MECHANICAL POWERS. 

Mrs. 7>. In this manner (fig. 6.) ; now if you observe 
the dotted circle which the handle describes in winding up 
the rope, you will perceive that the branch of the handle A, 
which is united to the axle, represents the spoke of a wheel, 
and answers the purpose of an entire wheel ; the other 
branch B affords no mechanical aid, merely serving as a han- 
dle to turn the wheel. 

Wheels are a very essential part to most machines : they 
are employed in various ways ; but, when fixed to the axle, 
their mechanical power is always the same ; that is, as the 
circumference of the wheel exceeds that of the axle, so much 
will the energy of its power be increased. 

Caroline. Then the larger the wh^el the greater must be 
its effect. 

]\Irs. B. Certainly. If you have ever seen any consid- 
erable mills or manufactures, you must hive admired the 
immense wheel, the revolution of which puts the whole of the 
machinery into motion ; and though so great an effect is 
produced by it, a horse or two has sufficient power to turn it ; 
sometimes a stream of water is used for tliat purpose, but of 
late years, a steam-engine has been found both the most pow- 
erful and the most convenient mode of turning the wheel. 

Caroline, Do not the vanes of a windmill represent a 
wheel, Mrs. B. ? 

Mrs. B. Yes ; and in this instance we have the advan- 
tage of a gratuitous force, the wind, to turn the wheel. One 
of the great benefits resulting from the use of machinery is, 
that it gives us a sort of empire over the powers of nature, 
and enables us to make them perform the labour which would 
otherwise fall to the lot of man. When a current of wind, a 
stream of water, or the expansive force of steam, performs our 
task, we have only to superintend and regulate their operations. 

The fourth mechanical power is the inclined plane ; this 
is nothing more than a slope, or declivity, frequently used to 
facilitate the drawing up of weights. It is not difficult to 
understand, that a weight may much more easily be draw^n 
up a slope than it can be raised the same height perpendicu- 
larly. But in this, as well as the other mechanical powers, 
the facility is purchased by a loss of time (fig. 7.) ; for the 
weight, instead of moving directly from A to C, must move 
from B to C, and as the length of the plane is to its height, so 
much is the resistance of the weight diminished. 



ON THE MECHANICAL POWERS. io 

Emily, Yes ; for the resistance, instead of being confined 
to the short line A C, is spread over the long line B C. 

Mrs. B, The wedge, which is the next mechanical pow- 
er, is composed of two inclined planes (^fig. 8.) : you may 
have seen wood-cutters use it to cleave wood. The resistance 
consists in the cohesive attraction of the wood, or any other 
body which the wedge is employed to separate ; and the 
advantage gained by this power is in the proportion of half 
its width to its length ; for whik the wedge forces asunder the 
coherent particles of the wood to A and B, it penetrates 
downwards as far as C. 

Emily, The wedge, then, is rather a compound than a 
distinct mechanical power, since it is composed of two 
inclined planes. 

Mrs. B. It is so. All cutting instruments are constructed 
upon the principle of the inclined plane, or the wedge : those 
that have but one edge sloped, like the chisel, may be referred 
to the inclined plane ; whilst the axe, the hatchet, and the 
knife (when used to split asunder) are used as wedges. 

Caroline. But a knife cuts best when it is drawn across 
the substance it is to divide. We use it thus in cutting meat, 
we do not chop it to pieces. 

Mrs. B. The reason of this is, that the edge of a knife is 
really a very fine saw, and therefore acts best when used like 
that instrument. 

The screw, which is the last mechanical power, is more 
complicated than the others. You will see by this figure, 
(fig. 9.) that it is composed of two parts, the screw and the 
nut. The screw S is a cylinder, with a spiral protuberance 
coiled round it, called the thread ; the nut N is perforated to 
contain the screw, and the inside of the nut has a spiral 
groove made to fit the spiral thread of the screw. 

Caroline. It is just like this little box, the lid of which 
screws on the box as you have described : but what is this 
handle which projects from the nut ? 

Mrs, B. It is a lever, which is attached to the nut, without 
which the screw is never used as a mechanical power : the 
nut with a lever L attached to it, is commonly called a winch. 
The power of the screw, complicated as it appears, is refera- 
ble to one of the most simple of the mechanical powers ; 
which of them do you think it is ? 

Caroline. In appearance, it most resembles the wheel 
and axle. 

7 



74 ON THE MECHANICAL POWERS, 

Mrs. B. The lever, it is true, has the effect of a wheel, as 
it is the means by which you wind the nut round ; but the 
lever is not considered as composing a part of the screw, 
though it is true, that it is necessarily attached to it. But 
observe, that the lever, considered as a wheel, is not fastened 
to the axle or screw, but moves round it, and in so doing, the 
nut either rises or descends, according to the way in which 
you turn it. 

Emily, The spiral thread of the screw resembles, I think, 
an inclined plane : it is a sort of slope, by means of which 
the nut ascends more easily than it would do if raised perpen- 
dicularly ; and it serves to support it when at rest. 

Mrs, B. Very well : if you cut a slip of paper in the 
form of an inclined plane, and wind it round your pencil, 
which will represent the cylinder, you will find that it makes 
a spiral line, corresponding to the spiral protuberance of the 
screw, (fig. 10.) 

Emily, Very true; the nut then ascends an inclined 
plane, but ascends it in a spiral, instead of a straight line ; the 
closer the thread of the screw, the more easy the ascent ; it 
is like having shallow, instead of steep steps to ascend. 

Mrs. B, Yes, excepting that the nut takes no steps, it 
gradually winds up or down ; then observe, that the closer 
the threads of the screw, the greater the number of revolu- 
tions the winch must make ; so that we return to the old 
principle, — what is saved in power is lost in time. 

Emily. Cannot the power of the screw be increased also, 
by lengthening the lever attached to the nut ? 

Mrs. B, Certainly. The screw, with the addition of 
the lever, forms a very powerful machine, employed either 
for compression or to raise heavy weights. It is used by 
book-binders, to press the leaves of books together ; it is used 
also in cider and wine presses, in coining, and for a variety of 
other purposes. 

All machines are composed of one or more of these six 
mechanical powers we have examined : I have but one more 
remark to make to you relative to them, which is, that friction 
in a considerable degree diminishes their force, allowance 
must therefore always be made for it in the construction of 
machinery. 

Caroline. By friction, do you mean one part of the ma- 
chine rubbing against another part contiguous to it. 

Mrs. B. Yes ; friction is the resistance which bodies 



ON THE MECHANICAL POWERS. 75 

meet with in rubbing against each other ; there is no such 
thing as perfect smoothness or evenness in nature : poHshed 
metals, though they wear that appearance, more than any 
other bodies, are far from really possessing it ; and their 
inequalities may frequently be perceived through a good 
magnifying glass. When, therefore, the surfaces of the two 
bodies come into contact, the prominent parts of the one will 
often fall into the hollow parts of the other, and occasion 
more or less resistance to motion. 

Caroline. But if a machine is made of polished metal, as 
a watch, for instance, the friction must be very trifling ? 

Mrs. B. In proportion as the surfaces of bodies are well 
polished, the friction is doubtless diminished ; but it is always 
considerable, and it is usually computed to destroy one- 
third of the power of a machine. Oil or grease is used to 
lessen friction ; it acts as a polish by filling up the cavities of 
the rubbing surfaces, and thus making them slide more easily 
over each other. 

Caroline. Is it for this reason that wheels are greased, 
and the locks and hinges of doors oiled ? 

Mrs. B. Yes ; in these instances the contact of the rub- 
bing surfaces is so close, and the rubbing so continual, that 
notwithstanding their being polished and oiled, a considera- 
ble degree of friction is produced. 

There are two kinds of friction ; the one occasioned by 
the sliding of the flat surface of a body, the other by the 
rolling of a circular body ; the friction resulting from the 
first is much the most considerable, for great force is required 
to enable the sliding body to overcome the resistance which 
the asperities of the surfaces in contact oppose to its motion, 
and it must be either lifted over, or break through them ; 
whilst, in the other kind of friction, the rough parts roll over 
each other with comparative facility ; hence it is, that wheels 
are often used for the sole purpose of diminishing the resist- 
ance of friction. 

Emily. This is one of the advantages of carriage-wheels ; 
is it not ? 

Mrs. B. Yes ; and the larger the circumference of the 
wheel the more readily it can overcome any considerable 
obstacles, such as stones, or inequalities in the road. When, 
in descending a steep hill, we fasten one of the wheels, we 
decrease the velocity of the carriage, by increasing the 
friction. 



76 ON THE MECHANICAL POWERS. 

Caroline, That is to say, by converting the rolling fric- 
tion into the dragging friction. And when you had casters 
put to the legs of the table, in order to move it more easily, 
you changed the dragging into the rolling friction. 

Mrs, B, There is another circumstance which we have 
already noticed, as diminishing the motion of bodies, and 
which greatly affects the power of machines. This is the 
resistance of the medium in which a machine is worked. All 
fluids, whether of the nature of air, or of water, are called 
mediums ; and their resistance is proportioned to their den- 
sity ; for the more matter a body contains, the greater the 
resistance it will oppose to the motion of another body 
striking against it. 

Emily, It would then be much more difficult to work a 
machine under water than in the air ? 

Mrs, B, Certainly, if a machine could be worked in 
vacuOy and without friction, it would be perfect ; but this is 
unattainable ; a considerable reduction of power must there- 
fore be allowed for the resistance of the air. 

We shall here conclude our observations on the mechan- 
ical powers. At our next meeting I shall endeavour to give 
you an explanation of the motion of the heavenly bodies. 



CONVERSATION VI. 



CAUSES OF THE EARTH'S ANNUAL MOTION. 

Of the Planets^ and their Motion ; Of the Diurnal Motion 
of the Earth and Planets, 



CAROLINE. 

I AM come to you to-day quite elated with the spirit of 
opposition, Mrs. B. ; for I have discovered such a powerful 
objection to your theory of attraction, that I doubt whether 
even your conjuror Newton, with his magic wand of attrac- 
tion, will be able to dispel it. 

Mrs, B. Well, my dear, pray what is this weighty objec- 
tion ? 

Caroline. You say that bodies attract in proportion to 
the quantity of matter they contain, now we all know the sun 
to be much larger than the earth : why, therefore, does it not 
attract the earth ; you will not, I suppose, pretend to say that 
we are falling towards the sun ? 

Emily, However plausible your objection appears, Caro- 
line, I think you place too much reliance upon it : when any 
one has given such convincing proofs of sagacity and wisdom 
as Sir Isaac Newton, when we find that his opinions are uni- 
rersally received and adopted, is it to be expected that any 
objection we can advance should overturn them ? 

Caroline, Yet I confess that I am not inclined to yield 
implicit faith even to opinions of the great Newton : for what 
purpose are we endowed with reason, if we are denied the 
privilege of making use of it, by judging for ourselves ? 

MrSy B, It is reason itself which teaches us, that when 

we, novices in science, start objections to theories established 

by men of acknowledged wisdom, we should be diffident 

rather of our own than of their opinion. I am far from wish- 

7* 



78 CAUSES OF THK 

ing to lay the least restraint on your questions ; you cannot 
be belter convinced of the truth of a system, tlian by finding 
that it resists all your attacks, but I would advise you not to 
advance your objections with so much confidence, in order 
that the discovery of their fallacy may be attended with less 
mortification. In answer to that you have just proposed, I 
can only say, that the earth really is attracted by the sun. 

Caj'oline. Take care at least that we are not consumed 
by him, Mrs. B. 

Mrs. B, We are in no danger ; but our magician Newton, 
as you are pleased to call him, cannot extricate himself from 
this difficulty without the aid of some cabalistical figures, 
which I must draw for him. 

Let us suppose the earth, at its creation, to have been 
projected forwards into universal space : we know that if no 
obstacle impeded its course it would proceed in the same 
direction, and with a uniform velocity for ever. In fig. 1 
plate 6., A represents the earth, and S the sun. We shall 
suppose the earth to be arrived at the point in which it is 
represented in the figure, having a velocity which would carry 
it on to B in the space of one month ; whilst the sun's attrac- 
tion would bring it to C in the same space of time. Observe 
that the two forces of projection and attraction do not act in 
opposition, but perpendicularly, or at a right angle to each 
other. Can you tell me now, how the earth will move ? 

Emily. I recollect your teaching us that a body acted 
upon by two forces perpendicular to each other would move 
in the diagonal of a parallelogram ; if, tlierefore, I complete 
the parallelogram by drawing the lines C D, B D, the earth 
will move in the diagonal A D. 

Mrs. B. A ball struck by two forces acting perpendicu- 
larly to each other, it is true, moves in the diagonal of a 
parallelogram ; but you must observe that the force of attrac- 
tion is continually acting upon our terrestrial ball, and produ- 
cing an incessant deviation from its course in a right line, 
which converts it into that of a curve line ; every point of 
w^hich may be considered as constituting the diagonal of an 
infinitely small parallelogram. 

Let us detain the earth a moment at the point D, and con- 
sider how it will be aflected by the combined action of the two 
forces in its new situation. It still retains its tendency to fly 
off in a straight line ; but a straight line would now carry it 



PIATE U 




^' 




^4^ 







^' 



earth's annual motion. 79 

away to F, whilst the sun would attract it in the direction 
D S ; how then will it proceed ? 

Emily. It will go on in a curve line, in a direction between 
that of the two forces. 

Mrs, B. In order to know exactly what course the earth 
will follow, draw another parallelogram similar to the first, in 
which the line D F describes the force of projection, and the 
line D S, that of attraction ; and you will find that the earth 
will proceed in the curve line D G. 

Caroline, You must now allow me to draw a parallelo- 
gram, Mrs. B. Let me consider in w^hat direction will the 
force of projection now impel the earth. 

Mrs, B. First draw a line from the earth to the sun 
representing the force of attraction ; then describe the force of 
projection at a right angle to it. 

Caroline, The earth will then move in the curve G I, of 
the parallelogram G H I K. 

Mrs, B, You recollect that a body acted upon by two 
forces, moves through a diagonal in the same time that it 
would have moved through one of the sides of the parallelo- 
gram, were it acted upon by one force only. The earth has 
passed through the diagonals of these three parallelograms in 
the space of three months, and has performed one quarter of 
a circle ; and on the same principle it will go on till it has 
completed the whole of the circle. It will then recommence 
a course, which it has pursued ever since it first issued from 
the hand of its Creator, and which there is every reason to 
suppose it will continue to follow, as long as it remains in 
existence. 

Emily, What a grand and beautiful effect resulting from 
so simple a cause ! 

Caroline, It affords an example, on a magnificent scale, 
of the circular motion which you taught us in mechanics. 
The attraction of the sun is the centripetal force, which con- 
fines the earth to a centre ; and the impulse of projection the 
centrifugal force, which impels the earth to quit the sun and 
fly off in a tangent. 

Mrs, B, Exactly so. A simple mode of illustrating the 
effect of these combined forces on the earth, is to cut a slip of 
card in the form of a right angle, (fig. 2. plate YI.) to describe 
a small circle at the angular point representing the earth, and 
to fasten the extremity of one of the legs of the angle to a 
fixed point, which we shall consider as the sun. Thus situa- 



30 CAUSES OF THE 

tedy the angle will represent both the centrifugal and centri- 
petal forces ; and if you draw it round the fixed point, you 
will see how the direction of the centrifugal force varies, 
constantly forming a tangent to the circle in which the earth 
moves, as it is constantly at a right angle with the centripetal 
force. 

Emihj, The earth, then, gravitates towards the sun with- 
out the slightest danger either of approaching nearer or 
receding further from it. How admirable this is contrived ! 
If the two forces which produce this circular motion had not 
been so accurately adjusted, one would ultimately have pre- 
vailed over the other, and we should either have approached 
so near the sun as to have been burnt, or have receded so far 
from it as to have been frozen. 

Mrs. B, What will you say, my dear, when I tell you 
that these two forces are not, in fact, so proportioned as to 
produce circular motion in the earth ? 

Caroline. You must explain to us, at least, in what man- 
ner we avoid the threatened destruction. 

Mrs. B. Let us suppose that when the earth is at A, 
(fig. 3.) its projectile force should not have given it a velocity 
sufficient to counterbalance that of gravity, so as to enable 
these powers conjointly to carry it round the sun in a circle ; 
the earth, instead of describing the line A C, as in the former 
figure, will approach nearer the sun in the line A B. 

Caroline. Under these circumstances, I see not what is 
tc prevent our approaching nearer and nearer the sun till we 
fall into it : for its attraction increases as we advance towards 
it, and produces an accelerated velocity in the earth which 
increases the danger. 

Mrs. B. And there is yet another danger, of which you 
are not aware. Observe, that as the earth approaches the sun, 
the direction of its projectile force is no longer perpendicular 
to that of attraction, but inclines more nearly to it. When 
the earth reaches that part of its orbit at B, the force of pro- 
jection would carry it to D, which brings it nearer the sun 
instead of bearing it away from it. 

Emily. If, then, we are driven by one power and drawn 
by the other to this centre of destruction, how is it possible for 
us to escape ? 

Mrs. B. A little patience, and you will find that we are 
not without resource. The earth continues approaching tlie 
sun with a uniformly increasing accelerated motion, till it 



earth's annual 3I0TI0N. 81 

ueaches the point E ; in what direction will the projectile 
ibrce now impel it ? 

Emily, In the direction E F. Here then the two forces 
act perpendicularly to each other, and the earth is situated 
just as it was in the preceding figure ; therefore, from this 
point, it should revolve round the sun in a circle. 

Mrs, B. No, all the circumstances do not agree. In mo- 
tion round a centre, 3^ou recollect that the centrifugal force 
increases with the velocity of the body, or, in other words, 
the quicker it moves the stronger is its tendency to fly oil in 
a right line. When the earth, therefore, arrives at E, its 
accelerated motion will have so far increased its velocity, and 
consequently its centrifugal force, that the latter will prevail 
over the force of attraction, and drag the earth away from the 
sun till it reaches G. 

Caroline, It is thus, then, that vre escape from the dan- 
gerous vicinity of the sun ; and in proportion as we recede 
from it, the force of its attraction, and, consequently, the velo- 
city of the earth's motion are diminished. 

Mrs, B, Yes. From G the direction of projection is 
towards H, that of attraction towards S, and the earth pro- 
ceeds between them with a uniformly retarded motion, till it 
has completed its revolution. Thus you see, that the earth 
travels round the sun, not in a circle, but an ellipsis, of which 
the sun occupies one of the foci ; and that in its course the 
earth alternately approaches, and recedes from it, without any 
danger of being either swallowed up, or being entirely carried 
away from it. 

Caroline, And I observe, that what I apprehended to be 
a dangerous irregularity, is the means by which the most per- 
fect order and harmony are produced ! 

Emily, The earth travels, then, at a very unequal rate, 
its velocity being accelerated as it approaches the sun, and 
retarded as it recedes from it. 

Mrs, B, It is mathematically demonstrable, that, in mov- 
ing round a point towards which it is attracted, a body passes 
over equal areas in equal times. The whole of the space 
contained Vv'ithin the earth's orbit, is, in fig. 4., divided into a 
number of areas, or spaces, 1, 2, 3, 4, &c. all of which are of 
equal dimensions, though of very different forms ; some of 
them, you see, are long and narrow, others broad and short : 
but they each of them contain an equal quantity of space. 
An imaginary line drawn from the centre of the earth to that 



^2 CAUSES OF THE 

of the sun, and keeping pace with the earth in its revolution, 
passes over equal areas in equal times ; that is to say, if it is 
a month going from A to B, it will be a month going from B 
to C, and another from C to E, and so on. 

Caroline. What long journeys the earth has to perform 
in the course of a month, in one part of her orbit, and how 
short they are in the other part ! 

Mrs, B. The inequality is not so considerable as appears 
in this figure ; for the earth's orbit is not so eccentric as it is 
there described ; and, in reality, differs but litde from a cir- 
cle ; that part of the earth's orbit nearest the sun is called its 
perihelion, that part most distant from the sun its aphelion ; 
and the earth is above three millions of miles nearer the sun 
at its perihelion than at its aphelion. 

Emily. I think I can trace a consequence from these dif- 
ferent situations of the earth ; is it not the cause of summer 
and winter ? 

Mrs. B. On the contrary ; during the height of summer, 
the earth is in that part of its orbit which is most distant from 
the sun, and it is during the severity of winter, that it ap- 
proaches nearest to it. 

Emily. That is very extraordinary ; and how then do 
you account for the heat being greatest, when we are most 
distant from the sun ? 

Mrs. B. The difference of the earth's distance from the 
sun in summer and winter, when compared with its total 
distance from the sun, is but ipconsiderable. The earth, it is 
true, is above three millions of miles nearer the sun in winter 
than in summer ; but that distance, however great it at first 
appears, sinks into insignificance in comparison of 95 mil- 
lions of miles, which is our mean distance from the sun. The 
change of temperature, arising from this difference, would 
scarcely be sensible ; were it not completely overpowered by 
other causes which produce the variations of the seasons ; but 
these I shall defer explaining, till we have made some further 
observations on the heavenly bodies. 

Caroline. And should not the sun appear smaller in sum- 
mer, when it is so much further from us ? 

Mrs. B. It actually does when accurately measured ; but 
the apparent difference in size, is, I believe, not perceptible to 
the naked eye. 

Emily, Then, since the earth moves with greatest velo- 
city in that part of its orbit nearest the sun, it must have 



EARTH'S ANNUAL MOTION. 83 

rdmpleted its journey through one half of its orbit in a shortev 
time than the other half ? 

Mrs. B. Yes, it is about seven days longer performing 
the summer-half of its orbit, than the winter-half. 

The revolution of all the planets round the sun is the result 
of the same causes, and is performed in the same manner as 
that of the earth. 

Caroline. Pray what are the planets ? 

Mrs. B, They are those celestial bodies, which revolve 
iike our earth about the sun ; they are supposed to resemble 
the earth also in many other respects ; and we are led by 
analogy to suppose them to be inhabited worlds. 

Caroline. I have heard so ; but do you not think such an 
opinion too great a stretch of the imagination ? 

Mrs. B. Some of the planets are proved to be larger than 
the earth ; it is only their immense distance from us, which 
renders their apparent dimensions so small. Now, if we 
consider them as enormous globes, instead of small twinkhng 
spots, we shall be led to suppose, that the Almighty would 
not have created them merely for the purpose of giving us a 
little light in the night, as it was formerly imagined, and we 
should find it more consistent with our ideas of the Divine 
wisdom and beneficence to suppose that these celestial bodies, 
should be created for the habitation of beings, who are, like 
us, blessed by his providence. Both in a moral as well as a 
physical point of view, it appears to me more rational to con- 
sider the planets as worlds revolving round the sun ; and the 
fixed stars as other suns, each of them attended by their 
respective system of planets, to which they impart their 
influence. We have brought our telescopes to such a degree 
of perfection, that from the appearances which the moon 
exhibits when seen through them, we have very good reason 
to conclude, that it is a habitable globe, for though it is true, 
that we cannot discern its towns and people, we can plainly 
perceive its mountains and valleys ; and some astronomers 
have gone so far as to imagine they discovered volcanos. 

Emily. If the fixed stars are suns, with planets revolving 
round them, why should we not see those planets as well as 
their suns ? 

Mrs. B. In the first place, we conclude that the planets 
of other systems, (like those of our own,) are much smaller 
than the suns which give them light ; therefore at so great a 
distance as to make the suns appear like fixed stars, the 



84 CAUSES OF TH£ 

planets would be quite invisible. Secondiyj the light of the 
planets being only reflected light, is much more feeble than 
that of the fixed stars. There is exactly the same difference 
as between the light of the sun and that of the moon ; the first 
being a fixed star, the second a planet. 

Emily, But if the planets are worlds like our earth, they 
are dark bodies ; and instead of shining by night, we should 
see them only by day-light. And why do we not see the 
fixed stars also by day-light ? 

Mrs, B, Both for the same reason ; their light is so faint, 
compared to that of our sun reflected by the atmosphere, that 
it is entirely efiaced by it : the light emitted by the fixed stars 
may probably be as strong as that of our sun, at an equal 
distance ; but being so much more remote, it is diflfused over 
a greater space, and is consequently proportionally weakened. 

Caroline, True ; I can see much better by the light of a 
candle that is near me, than by that of one at a great distance. 
But I do not understand what makes the planets shine ? 

Mrs, B* What is it that makes the steel buttons on your 
brother's coat shine ? 

Caroline, The sun. But if it was the sun which made 
the planets shine, we should see them in the day-time when 
the sun shone upon them ; or if the faintness of their light 
prevented our seeing them in the day, we should not see 
tliem at all, for the sun cannot shine upon them in the night. 

Mrs, B, There you are in error. But in order to explain 
this to you, I must first make you acquainted with the various 
motions of the planets. 

You know, that according to the laws of attraction, the 
planets belonging to our system all gravitate towards the sun ; 
and that this force combined with that of projection, will 
occasion their revolution round the sun, in orbits more or less 
elliptical, according to the proportion which these two forces 
bear to each other. 

But the planets have also another motion ; they revolve 
upon their axes. The axis of a planet is an imaginary line 
which passes through its centre, and on which it turns ; and 
it is this motion which produces day and night. With that 
side of the planet facing the sun it is day ; and with the 
opposite side, which remains in darkness it is night. Our 
earth, which we consider as a planet, is 24 hours in performing 
one revolution on its axis ; in that period of time, therefore, 
we have a day and a night ; hence this revolution is called 



earth's annual motion. 85 

the earth^s diurnai or daily motion ; and it is this revolution 
of the earth from west to east which produces an apparent 
motion of the sun^ moon, and stars in a contrary direction. 

Let us now suppose ourselves to be beings independent of 
any planet, travelling in the skies, and looking upon the earth 
in the same point of view as upon the other planets. 

Caroline. It is not flattering to us, its inhabitants, to see 
it make so insignificant an appearance. 

Mrs. B. To those who are accustomed to contemplate 
it in this light, it never appears more glorious. We are 
taught by science to distrust appearances : and instead of 
considering the planets as little stars, we look upon them 
either as brilliant suns or habitable worlds, and we consider 
the whole together as forming one vast and magnificent sys- 
tem, worthy of the Divine hand by which it was created. 

Emily. I can scarcely conceive the idea of this immensity 
of creation ; it seems too sublime for our imagination : — and 
to think that the goodness of Providence extends over mil- 
lions of worlds throughout a boundless universe — Ah ! Mrs. 
B., it is we only who become trifling and insignificant beings 
in so magnificent a creation ! 

Mrs. B. This idea should teach us humility, but without 
producing despondency. The same Almighty hand which 
guides these countless worlds in their undeviating course, 
conducts with equal perfection the blood as it circulates 
through the veins of a fly, and opens the eye of the insect to 
behold His wonders. Notwithstanding this immense scale of 
creation, therefore, w^e need not fear to be disregarded or 
forgotten. 

But to return to our station in the skies. We were, if you 
recollect, viewing the earth at a great distance, in appearance 
a little star, one side illuminated by the sun, the other in 
obscurity. But would you believe it, Caroline, many of the 
inhabitants of this little star imagine that when that part 
which they inhabit is turned from the sun, darkness prevails 
throughout the universe merely because it is night with them ; 
whilst, in reality, the sun never ceases to shine upon every 
planet. When, therefore, these little ignorant beings look 
around them during their night, and behold all the stars shin- 
ing, they cannot imagine why the planets, which are dark 
bodies, should shine, concluding, that since the sun does not 
illumine themselves, the whole universe must be in darkness. 
Caroline. I confess that I was one of these ignorant peo- 



86 CAUSES OF THE EARTh's ANNUAL MOTION. 

pie ; but I am now very sensible of the absurdity of such an 
idea. To the inhabitants of the other planets, then, we must 
appear as a little star ? 

Mrs. B, YeSj to those which revolve round our sun ; 
for since those which may belong to other systems (and 
whose existence is only hypothetical,) are invisible to us, it is 
probable, that we also are invisible to them. 

Emily, But they may see our sun as we do theirs, in ap- 
pearance a fixed star ? 

Mrs, B, No doubt, if the beings who inhabit those planets 
are endowed with senses similar to ours. By the same rule, 
we must appear as a moon, to the inhabitants of our moon ; 
but on a larger scale, as the surface of the earth is about 
thirteen times as large as that of the moon. 

Emily, The moon, Mrs. B., appears to move in a differ- 
ent direction, and in a different manner from the stars ? 

Mrs, B, I shall defer the explanation of the motion of the 
moon, till our next interview, as it would prolong our present 
lesson too much. 



CONVERSATION VII 



ON THE PLANETS. 



fjf the Satellites or Moom ; Gravitij diminishes as the 
Square of the distance ; Of the Solar System; Of Com- 
ets ; Constellations. Signs of the Zodiac; Of Coperni- 
rus^ Newfon* &'c. 



MRS.B. 

The planets are distinguished into primary and secondary. 
Those which revolve immediately about the sun are called 
primary. Many of these are attended in their course by 
smaller planets, which revolve round them : these are called 
secondary planets, satellites, or moons. Such is our moon 
which accompanies the ^arth, and is carried with it round the 
sun. 

Emily, How then can you reconcile the motion of the 
secondar}' planets to the laws of gravitation ; for the sun is 
much larger than any of the primary planets ; and is not the 
power of gravity proportional to the quantity of matter ? 

Caroline, Perhaps the sun, though much larger, may be 
less dense than the planets. Fire you know is very ligbt, 
and it may contain but little matter, though of great mag-.ii- 
tude. 

Mrs. B. We do not know of what kind of matter the sun 
is made ; but we may be certain, that since it is the general 
centre of attraction of our system of planets, it must be the 
body which contains the greatest quantity of matter in tiat 
system. 

You must recollect, that the force of attraction is not only 
proportional to the quantity of matter, but to the degree of 



88 ON THE PLANETS. 

proximity of the attractive body : this power is weakened, by 
being diffused, and diminishes as the squares of the distances 
increase. The square is the product of a number muhiplied 
by itseh^; so that a planet situated at twice the distance at 
which we are from the sun would gravitate four times less 
than we do ; for the product of two multiplied by itself is four. 

Caroline, Then the more distant planets move slower in 
their orbits; for their projectile force must be proportioned 
to that of attraction ? But I do not see how this accounts for 
the motion of the secondar}^ round the primary planets j in 
preference to the sun ? 

Emily. Is it not because the vicinity of the primary 
planets renders their attraction stronger than that of the sun ? 

Mrs, B, Exactly so. But since the attraction between 
bodies is mutual, the primary planets are also attracted by 
the satellites, which revolve round them. The moon attracts 
the earth, as well as the earth the moon ; but as the latter is 
the smaller body, her attraction is proportionally less ; 
therefore neither the earth revolves round the moon, nor the 
moon round the earth ; but they both revolve round a point, 
which is their common centre of gravity, and which is as 
much nearer the earth than the moon, as the gravity of the 
former exceeds that of the latter. 

Emily, Yes, I recollect your saying, that if two bodies 
were fastened together by a wire or bar, their common centre 
of gravity would be in the middle of the bar, pro^'ided the 
bodies were of equal weight ; and if they differed in weight, it 
would be nearer the larger body. If then the earth and moon 
had no projectile force which prevented their mutual attrac- 
tion from bringing them together, they would meet at their 
common centre of gravity. 

Caroline, The earth then has a great variety of motions, 
it revolves round the sun, upon its axis, and round the point 
towards which the moon attracts it. 

Mrs. B, Just so ; and this is the case with evory planet 
which is attended by satellites. The complicated effect of 
this variety of motions, produces certain irregularities, which, 
however, it is not necessary to notice at present. 

The planets act on the sun in the same manner as they are 
themselves acted on by their satellites ; for attraction, you 
must remember, is always mutual ; but the gravity of the 
planets (even when taken collectively) is so trifling compared 



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Ox\ THE PLANETS, ' 89 

with that of the sun, that they do not cause the latter to move 
so much as one half of his diameter. The planets do not, 
therefore, revolve round the centre of the sun, but round a 
point at a small distance from its centre, about which the sun 
also revolves. 

Emily, I thought the sun had no motion ? 

Mrs. B, You were mistaken ; for besides that which I 
have just mentioned, which is indeed very inconsiderable, he 
revolves on his axis ; this motion is ascertained by observing 
certain spots w^hich disappear, and re-appear regularly at 
stated times. 

Caroline, A planet has frequently been pointed out to 
me in the heavens ; but I could not perceive that its motion 
differed from that of the fixed stars, which only appear to 
move. 

Mrs, B, The great distance of the planets renders their 
motion apparently so slow, that the eye is not sensible of 
their progress in their orbit, unless we watch them for some 
considerable length of time : in different seasons they appear 
in different parts of the heavens. The most accurate idea I 
can give you of the situation and motion of the planets, will 
be by the examination of this diagram, (plate VII. fig. 1.) 
representing the solar system, in which you will find every 
planet with its orbit delineated. 

Emily, But the orbits here are all circular, and you said^- 
that they were elliptical. The planets appear too, to be 
moving round the centre of the sun ; whilst you told us that 
they moved round a point at a little distance from thence. 

Mrs, B, The orbits of the planets are so nearly circular, 
and the common centre of gravity of the solar system so near 
the centre of the sun, that these deviations are scarcely worth 
observing. The dimensions of the planets, in their true pro- 
portions, you will find delineated in fig. 2. 

Mercury is the planet nearest the sun ; his orbit is conse- 
quently contained within ours ; but his vicinity to the sun, 
occasions his being nearly lost in the brilliancy of his rays ; 
and when we see the sun, he is so dazzling, that very accurate 
observations cannot be made upon Mercury. He performs 
his revolution round the sun in about 87 days, which is conse- 
quently the length of his year. The time of his rotation on 
his axis is not known ; his distance from the sun is computed 
to be 37 millions of miles, and his diameter 3180 miles, 
8* 



90 ON THE PLANETS. 

The heat of this planet is so great, that water cannot exist 
tliere, but in a state of vapour, and metals would be liquified.^ 

Caroline. Oh, what a dreadful climate ! 

Mrs, B. Though we could not live there, it may be per- 
fectly adapted to other beings destined to inhabit it. 

Venus, the next in the order of planets, is 68 millions of 
miles from the sun ; she revolves about her axis in 23 hours 
and 21 minutes, and goes round the sun in 244 days 17 hours. 
The orbit of Venus is also within ours ; during one half of 
her course in it, we see her before sun-rise, and she is called 
the morning star ; in the other part of her orbit, she rises 
later than the sun. 

Caroline. In that case, we cannot see her, for she must 
rise in the day time ? 

Mrs. B. True ; but when she rises later than the sun, 
sjie also sets later ; so that we perceive her approaching the 
horizon after sun-set : she is then called Hesperus, or the 
evening star. Do you recollect those beautiful lines of Milton : 

Now came still evening on, and twilight gray 
Had in her sober livery all things clad ; 
Silence accompanied ; for beast and bird, 
They to their grassy couch, these to their nests 
"Were slunk, all but the wakeful nightingale ; 
She all night long her amorous descant sung ; 
Silence waspleas'd ; now glow'd the firmament 
With living sapphires. Hesperus, that led 
The starry host, rode brightest, till the moon 
Rising in clouded majesty, at length 
Apparent queen unveil'd her peerless light, 
And o'er the dark her silver mantle threw. 

The planet next to Venus is the Earth, of which we shall 
soon speak at full length. At present I shall only observe, 
that we are 95 millions of miles distant from the sun, that we 
perform our annual revolution in 365 days, 5 hours, and 49 
minutes ; and are attended in our course by a single moon. 

Next follows Mars. He can never come between us and 
the sun, like Mercury and Venus ; his motion is, however, 
very perceptible, as he may be traced to different situations in 

* The intenseness of the sun's heat, which is in the same proportion as 
his light, is seven times as great in Mercur} as with us ; so that water 
there would be carried off in the shape of steam, for by experiments with 
the thermometer, it appears that a heat seven times greater than that d" the 
sun's beams in summer will serve to make water boil. 



ON THE PLANETS. 91 

the heavens; his distance from the gun is 144 miUions of 
miles ; he turns round his axis in 24 hours and 39 minutes ; 
and he performs his annual revolution, in about 687 of our 
days: his diameter is 4120 miles. Then follow four very 
small planets, Juno, Ceres, Pallas, and Vesta, which have 
been recently discovered, but whose dimensions and distances 
from the sun have not been very accurately ascertained.* 

Jupiter is next in order : this is the largest of all the plan- 
ets. He is about 490 millions of miles from the sun, and 
completes his annual period in nearly 12 of our years. He 
turns round his axis in about ten hours. He is above 1200 
times as big as our earth ; his diameter being 86^000 miles. 
The respective proportions of the planets cannot, therefore, 
you see, be conveniently delineated in a diagram. He is 
attended by four moons. t 

The next planet is Saturn, whose distance from the sun is 
about 900 millions of miles ; his diurnal rotation is performed 
in 10 hours and a quarter: — his annual revolution in nearly 
30 of our years. His diameter is 79,000 miles. This planet 
is surrounded by a luminous ring, the nature of which, as- 
tronomers are much at a loss to conjecture ; he has seven 
moons. Lastly, we observe the Georgium Sidus, discovered 
by Dr. Herschel, and which is attended by six moons. J 

Caroline. How charming it must be in the distant planets, 
to see several moons shining at the same time ; I think I 
should like to be an inhabitant of Jupiter or Saturn. 

Mrs. B. Not long, I believe. Consider what extreme 
cold must prevail in a planet, situated as Satarn is, at nearly 
ten times the distance at which we are from the sun. Then 
his numerous moons are far from making so splendid an 

*These anomalous bodies, so unlike the other primary planets, Dr. 
Herschel has denominated Asteroids. Probably they are the fragments 
of some planet ; or perhaps other similar bodies abound in the solar 
system, though they have hitherto, from their sraallness or darkness, 
escaped observation. 

-f Jupiter is surrounded by cloudy substances, subject to frequent 
changes in their situation and appearance, called Belts. These Belts are 
sometimes of a regular form ; sometimes interrupted and broken ; and 
sometimes not at all to be seen. 

t This ring is set edgewise round it, and the distance of the rmg from 
the planet is equal to the breadth of the ring. The sun shines for almost 
fifteen of our years together on the northern side of the ring ; then goes 
off, and shines as long on the southern side of it, so there is but one day 
and one night on each side of the ring, in the time of Saturn's whole 
revolution about the sun, whi«h takes up almost thirty of our years. 



92 ON THE PLANETS. 

appearance as ours ; for th^y can reflect only the light which 
they receive from the sun ; and both light and heat decrease 
in the same ratio or proportion to the distances as gravity. 
Can you tell me now how much more light we enjoy than 
Saturn. 

Caroline, The square of ten, is a hundred ; therefore, 
Saturn has a hundred times less — or to answer your question 
exactly, we have a hundred times more light and heat than 
Saturn — this certainly does not increase my wish to become 
one of the poor wretches who inhabit that planet.* 

Mrs, B. May not the inhabitants of Mercury, with equal 
plausibility, pity us, for the insupportable coldness of our 
situation ; and those of Jupiter and Saturn for our intolerable 
heat ? The Almighty Power w^hich created these planets, 
and placed them in their several orbits, has no doubt peopled 
them w^ith beings whose bodies are adapted to the various 
temperatures and elements in which they are situated. If we 
judge from the analogy of our own earth, or from that of the 
great and universal beneficence of Providence, we must con- 
clude this to be the case. 

Caroline. Are not comets also supposed to be planets ? 

Mrs. B. Yes, they are ; for by the re-appearance of some 
of them, at stated times, they are know^n to revolve round the 
sun, but in orbits so extremely eccentric, that they disappear 
for a great number of years. If they are inhabited, it must 
be by a species of beings very different, not only from the 
inhabitants of this, but from those of any of the other planets, 
as they must experience the greatest vicissitudes of heat and 
cold ; one part of their orbit. being so near the sun, that their 
heat, when there, is computed to be greater than that of red- 
hot iron ; in this part of its orbit, the comet emits a luminous 
vapour, called the tail, which it gradually loses as it recedes 
from the sun ; and the comet itself totally disappears from 
our sight, in the more distant parts of its orbit, which extends 
considerably beyond that of the furthest planet. 

The number of comets belonging to our system, cannot be 
ascertained, as some of them are whole centuries before they 
make their re-appearance. The numbers that are known by . 
their regular re-appearance is only three. 

Emily. Pray, Mrs. B. what are the constellations ? 

• The sun's light at Saturn is 1 0^0 times as great as the light of the 
full moon is to us. 



PhAT^ 1 I 













ON THE PLANETS. 93 

Mrs, B, They are the fixed stars, which the ancients, in 
order to recognize them, formed into groupes, and gave the 
names of the figures, which you find delineated on the celestial 
globe. In order to show their proper situations in the heav- 
ens, they should be painted on the internal surface of a hollow 
sphere, from the centre of which you should view them ; you 
would then behold them, as they appear to be situated in the 
heavens. The twelve constellations, called the signs of the 
zodiac, are those w^hich are so situated, that the earth in its 
annual revolution passes directly between them and the sun. 
Their names are Aries, Taurus, Gemini, Cancer, Leo, Virgo, 
Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces ; 
the whole occupying a complete circle, or broad belt, in the 
heavens, called the zodiac, (plate VIII. fig, 1.) Hence, a 
right line drawn from the earth, and passing through the sun, 
v>^ould reach one of these constellations, and the sun is said to 
be in that constellation at which the line terminates : thus, 
when the earth is at A, the sun would appear to be in the 
constellation or sign Aries ; when the earth is at B, the sun 
would appear in Cancer ; when the earth was at C, the sun 
would be in Libra ; and when the earth was at D, the sun 
would be in Capricorn. This circle, in which the sun thus 
appears to move, and which passes through the middle of the 
zodiac, is called the ecliptic. 

Caroline, But many of the stars in these constellations 
appear beyond the zodiac. 

Mrs, B, We have no means of ascertaining the distance 
of the fixed stars. When, therefore, they are said to be in the 
zodiac, it is merely implied, that they are situated in that 
direction, and that they shine upon us through that portion of 
the heavens, which we call the zodiac* 

Emihj, But are not those large bright stars, which are 
called stars of the first magnitude, nearer to us, than those 
small ones which vv^e can scarcely discern ? 

Mrs, B. It may be so ; or the difference of size and 
brilliancy of the stars may proceed from their difference of 
dimensions ; this is a point which astronomers are not enabled 
to determine. Considering them as suns, I see no reason 

* An easy distinction between a planet and a f.xed star is this — the 
torn-iCr shines with a steady liglit, but the latter is constantly twinkling. 
\V hat it is which occasions tliis twinkling* oi* scintillation of a star, y^t 
remains undecided. 



94 ON THE PLANETS. 

why different sunS should not vary in dimensions as well as 
the planets belonging to them. 

Emily, Whai a wonderful and beautiful system this is, and 
how astonishing to think that every fixed star may probably 
be attended by a similar train of planets ! 

Caroline, You will accuse me of being very incredulous, 
but I cannot help still entertaining some doubts, and fearing 
that there is more beauty than truth in this system. It cer- 
tainly may be so ; but there does not appear to me to be 
sufficient evidence to prove it. It seems so plain and obvious 
that the earth is motionless, and that the sun and stars revolve 
round it ; your solar system, you must allow, is directly in 
opposition to the evidence of our senses. 

Mrs, B. Our senses so often mislead us, that we should 
not place implicit reliance upon them. 

Caroline, On what then can we rely, for do we not receive 
all our ideas through the medium of our senses ? 

Mrs, B, It is true that they are our primary source of 
knowledge ; but the mind has the power of reflecting, judging, 
and deciding upon the ideas received by the organs of sense. 
This faculty, which we call reason, has frequently proved to 
us, that our senses are liable to err. If you have ever sailed 
on the water, with a very steady breeze, you must have seen 
the houses, trees, and every object move, while you were 
sailing. 

Caroline, I remember thinking so, when I was very 
young ; but I now know that their motion is only apparent. 
It is true that my reason, in this case, corrects the error of my 
sight. 

Mrs, B, It teaches you that the apparent motion of the 
objects on shore, proceeds from your being yourself moving, 
and that you are not sensible of your own motion, because 
you meet with no resistance. It is only when some obstacle 
impedes our motion, that we are conscious of moving ; and if 
you were to close your eyes when you were sailing on calm 
water, with a steady wind, you would not perceive that you 
moved, for 3^ou could not feel it, and you could see it only by 
observing the change of place of the objects on shore. So it 
is with the motion of the earth ; every thing on its surface, 
and the air that surrounds it, accompanies it in its revolution ; 
it meets w^ith no resistance : therefore, hke the crew of a 
vessel sailing w^ith a fair wind, in a calm sea, we are insensible 
of our motion. 



ON THE PLANETS. 95 

Caroline, But the principal reason why the crew of a 
vessel in a calm sea do not perceive their motion, is, because 
they move exceedingly slowly ; while the earth, you say, 
revolves with great velocity. 

Mrs* B. It is not because they move slowly, but because 
they move steadily, and meet with no irregular resistances, 
that the crew of a vessel do not perceive their motion ; for 
they would be equally insensible to it, with the strongest 
wind, provided it were steady, that they sailed with it, and 
that it did not agitate the water ; but this last condition, you 
know, is not possible, for the wind will always produce waves 
whkh offer more or less resistance to the vessel, and then the 
motion becomes sensible, because it is unequal. 

Caroline. But, granting this, the crew of a vessel have a 
proof of their motion, though insensible, which the inhabitants 
of the earth cannot have, — the apparent motion of the objects 
on shore. 

Mrs. B. Have we not a similar proof of the earth's motion, 
in the apparent motion of the sun and stars. Imagine the 
earth to be sailing round its axis, and successively passing by 
every star, which, like the objects on land, we suppose to be 
moving instead of ourselves. I have heard it observed by an 
aerial traveller in a balloon, that the earth appears to sink 
beneath the balloon, instead of the balloon rising above the 
earth. 

It is a law which we discover throughout nature and worthy 
of its great Author, that all its purposes are accomplished by 
the most simple means ; and what reason have we to suppose 
this law infringed, in order that we may remain at rest, while 
the sun and stars move round us ; their regular motions, 
which are explained by the laws of attraction on the first 
supposition, would he unintelligible on the last, and the order 
and harmony of the universe be destroyed. Think what an 
immense circuit the sun and stars would make daily, were 
their apparent motions real. We know many of them to be 
bodies more considerable than our earth ; for our eyes vainly 
endeavour to persuade us, that they are little brilliants spark- 
ling in the heavens, while science teaches us that they are 
immense spheres, whose apparent dimensions are diminished 
by distance. Why then should these enormous globes daily 
traverse such a prodigious space, merely to prevent the 
necessity of our earth's revolving on its axis ? 



^J ON THE PLANETS. 

Caroline. I think I must now be convinced. But you 
wiilj I hope, allow me a little time to familiarize myself to an 
idea so different from that which I have been accustomed to 
entertain. And pray, at what rate do we move ? 

Mrs. B. The motion produced by the revolution of the 
earth on its axis, is about eleven miles a minute, to an inhab- 
itant of London. 

Emily. But does not every part of the earth move with 
the same velocity ? 

Mrs. B. A moment's reflection would convince you of 
the contrary ; a person at the equator must move quicker 
dian one situated near the poles, since they both perform a 
revolution in 24 hours. 

Emily. True, the equator is farthest from the axis of 
motion. But in the earth's revolution round the sun^ every 
part must move with equal velocity ? 

Mrs. B. Yes, about a thousand miles a mini'te. 

Caroline. How astonishing ! — and that it should be pos- 
sible for us to be insensible of such a rapid motion. You 
would not tell me this sooner, Mrs. B., for fear f increasing 
my incredulity. 

Before the time of Newton, was not the earth supposed to 
be in the centre of the system, and the sun, moon, and stars to 
revolve round it ? 

Mrs. B. This was the system of Ptolemy in ancient 
times ; but as long ago as the beginning of the sixteenth cen- 
tury it was discarded, and the solar system, such as I have 
shown you, was established by the celebrated astronomer 
Copernicus, and is hence called the Copernican system. 
But the theory of gravitation, the source from which this 
beautiful and harmonious arrangement flows, we owe to the 
powerful genius of Newton, who lived at a much later period. 

Emily. It appears, indeed, far less difficult to trace by 
observation the motion of the planets, than to divine by what 
power they are impelled and guided. I wonder how the 
idea of gravitation could first have occurred to Sir Isaac 
Newton ? 

Mrs. B. It is said to have been occasioned by a circum- 
stance from which one should little have expected so grand a 
theory to have arisen. 

During the prevalence of the plague in the year l665, 
Newton retired into the country io avoid the contagion : when 



ON THE PLANETS. 97 

^^ikig one day in his orchard he observed an apple fail from 

tree, and was led to consider what could be the cause which 
jrought it to the ground. 

Caroline, If I dared to confess it, Mrs. B., I should say 
that such an enquiry indicated rather a deficiency than a. 
superiority of intellect. I do not understand how any one 
can wonder ai what is so natural and so common. 

Mr^. B. It is the mark of superior genius to find matter 
for wonder, observation, and research, in circumstances which, 
to the ordinary mind, appear trivial, because they are com- 
mon, and with which they are satisfied, because they are 
natural, without reflecting that nature is our grand field of 
observation, that within it is contained our whole store of 
knowledge ; in a word, that to study the works of nature, is 
to learn to appreciate and admire the wisdom of God. Thus, 
it was the simple circumstan€e of the fall of an apple, which 
led to the discovery of the laws upon which the Copernicaii 
system is founded ; and whatever credit this system had ob- 
tained before, it now rests upon a basis from which it cannot 
be shaken. 

Emily. This was a most fortunate apple, and more wor- 
t^.y to be commemorated than all those that have been sung 
by the poets. The apple of discord for which the goddesses 
contended ; the golden apples by which Atalanta won the 
race ; nay, even the apple which William Tell shot from the 
head of his son, cannot be compared to this ! 

The Strx. The sun is a spherical body, situated near tlie centre of 
gravity in the system of planets, of which our earth is one. Its diameter 
IS 877,547 English miles ; or equal to 100 diameters of the earth ; and 
therefore its cubic magnitude must exceed that of the earth one millioa 
of times. It revolves round its axis in 25 days, and 10 hours, vhich has 
been determined by means of several dark spots seen with telescopes on 
that luminary. Dr. Herschel supposes these spots in the sun to be the 
appearance of the opaque body of the sun througl^ the openings in his 
luminous atapoaphere,. 



CONVERSATION VIH- 



ON THE EARTH. 

Of the Terrestrial Globe ^ Of the Figure of the Earth ; Of 
the Pendulum ; Of the Variation of the Seasons^ and of 
the Length of Days and Nights ; Of the causes of the 
Heat of Summer ; Of Salary Siderial^ and Equal or Mean 
Time. 



MRS. B. 

As the earth is the planet in which we are the most partic- 
ularly interested, it is my intention this morning, to explain to 
you the effects resulting from its annual and diurnal motions ; 
but for this purpose it will be necessary to make j^ou acquaint- 
ed with the terrestrial globe : you have not either of you^ I 
conclude, learnt the use of the globes ?* 

Caroline. No ; I once indeed learnt by heart the names 
of the lines marked on the globe, but as I was informed they 
were only im,aginary divisions, they did not appear to me 
worthy of much attention, and were soon forgotten. 

Mrs* B» You suppose, then, that astronomers had been 
at the trouble of inventing a number of lines to little purpose. 
It will be impossible for me to explain to you the particular 
effects of the earth's motion without your having acquired a 
knowledge of these lines : in plate VIH. fig. 2. you will find 

• The earth is of a globular form. For, 1. The shadow of the earth 
projected on the moon in an eclipse is always circular ; which appearance 
could only be produced by a spherical body. 2. The convexity of the 
surface of the sea is evident ; the mast of an approaching ship being seea 
before its hull. 3. The north pole becomes more elevated by travelling 
northward, in proportion to the space passed over. 4. Navigators have 
sailed round the earth, and by steering their course continually weftward^ 
t^rriyed, at leogth^ at the plate from wheaoe they departed* 



ON THE JEARTH. 99 

them all delineated ; and you must learn them perfectly if 
you wish to make any proficiency in astronomy. 

Caroline, I was taught them at so early an age that I 
could not understand their meaning ; and I have often heard 
you say that the only use of words was to convey ideas. 

Mrs. B, The names of these lines would have conveyed 
ideas of the figures they were designed to express, though the 
use of these figures might at that time have been too difficult 
for you to understand. Childhood is the season when impres- 
sions on the memory are most strongly and most easily made : 
k is the period at which a large stock of ideas should be treas- 
ured up J the application of which we may learn when the 
understanding is more developed. It is, I think, a very mis- 
taken notion that children should be taught such things only^ 
as they can perfectly understand. Had you been early made 
acquainted with the terms which relate to figure and motion^ 
how much it would have facilitated your progiess in natural 
philosophy. I have been obliged to confine myself to the 
most common and familiar expressions, in explaining the laws 
of nature, though 1 am convinced that appropriate and scien- 
tific terms would have conveyed more precise and accurate 
ideas ; but I was afraid of not being understood. 

Emily. You may depend upon our learning the names of 
these lines thoroughly, Mrs* B. ; but before we commit them 
to memory, will you have the goodness to explain them to us ? 

Mrs. B, Most willingly. This globe, or sphere, repre- 
sents the earth ; the line which passes through its centre, and 
on which it turns, is called its axis, and the two extremities oi 
the axis A and B, are the poles, distinguished by the names 
of the north and south pole. The circle C D, which divides 
the globe into two equal parts between the poles, is called the 
equator, or equinoctial line ; that part of the globe to the 
north of the equator is the northern hemisphere ; that part 
to the south of the equator, the southern hemisphere. The 
small circle E F, which suj-rounds the north pole, is called 
the arctic circle ; that G H, which surrounds the south pole, 
the antarctic circle. There are two intermediate circles be- 
tween the polar circles and the equator ; that to the north, 
I Kj called the tropic of Cancer ; that to the south, L M, 
called the tropic of Capricorn. Lastly, this circle, L K, 
which divides the globe into two equal parts, crossing the 
equator and extending northward as far as the tropic of Can- 
cer, and southward as far as the tropic of Capricorn, is called 



100 ON THE EARTH. 

tlie ecliptic. The delineation of the ecliptic on the terrestrial 
globe is not without danger of conveying false ideas ; for the 
ecliptic (as I have before said) is an imaginary circle in the 
heavens passing through the middle of the zodiac, and situated 
in the plane of the earth's orbit. 

Cai'oline, I do not understand the meaning of the plane 
of the earth's orbit. 

Mrs, B. A plane, or plain, is an even level surface. Let 
us suppose a smooth thin solid plane cutting the sun through 
the centre, extending out as far as the fixed stars, and termin- 
ating in a circle which passes through the middle of the zodiac ; 
in this plane the earth would move in its revolution round the 
sun ; it is therefore called the plane of the earth's orbit, and 
the circle in which this plane cuts the signs of the zodiac is 
the ecliptic. Let the fig, 1» plate IX. represent such a plane, 
S the sun, E the earth with its orbit, and A B C D the echp- 
tic passing through the middle of the zodiac. 

Emily, li tlie ecliptic relates only to the heavens, why is 
it described upon the terrestrial globe ? 

Mrs, B, It is. convenient for the demonstration of a vari- 
ety of problems in the use of the globes ; and besides, the 
obliquity of this circle to the equator is rendered more con- 
spicuous by its being described on the same globe ; and the 
obliquity of the ecliptic shows the inclination of the earth's 
axis to the plane of its orbit. But to return to fig. 2. plate 
VIII. 

The spaces between the several parallel circles on the ter- 
restrial globe are called zones ; that which is comprehended 
between the tropics is distinguished by the name of the torrid 
zone ; the spaces which extend from the tropics to the polar 
circles, the north and south temperate zones ; and the spaces, 
contained within the polar circles, the frigid zones. 

The several lines which, you observe, are drawn from one 
pole to the other, cutting the equator at right angles, are called 
meridians. When EJjy one of these meridians is exactly 
opposite the sun it is rkid-day, or twelve o'clock in the day, 
with all the places situated on that meridian ; and, with the 
places situated on the opposite meridian, it is consequently 
midnight. 

Emihj, To places situated equally distant from these two 
meridians, it must then be six o'clock ? 

Mrs. B. Yes : if thev are to the east of the sun'$ meri- 



BLATJS iX. 




ON THE EARTH. lOi 

ciian it is six o'clock in the afternoon, because the sun will 
have previously passed over them ; if to the west, it is six 
o'clock in the morning, and the sun will be proceeding to-^ 
wards that meridian. 

Those circles which divide the globe into two equal parts^ 
such as the equator and the ecliptic, are called greater circles ; 
to distinguish them from those which divide it into two une- 
qual parts, as the tropics and polar circles, which are called 
lesser circles. All circles are divided into 360 equal parts, 
called degrees, and degrees into 60 equal parts, called minutes. 
The diameter of a circle is a right line drawn across it^ and 
passing through the centre ; for instance, the boundary of 
this sphere is a circle, and its axis the diameter of that circle ; 
the diameter is equal to a little less than one-third of the 
circumference. Can you tell me nearly how many degrees it 
contains ? 

Caroline, It must be something less than one-third of 
360 degrees, or nearly 120 degrees. 

Mrs, B, Right ; now Emily you may tell me exactly 
how many degrees are contained in a meridian ? 

Emily. A meridian, reaching from one pole to the other, 
is half a circle, and must therefore contain 180 degrees. 

Mrs, B, Very well ; and what number of degrees are 
there from the equator to the poles ? 

Caroline, The equator being equally distant from either 
pole, that distance must be half of a meridian, or a quarter of 
the circumference of a circle, and contain 90 degrees. 

Mrs, B, Besides the usual division of circles into degrees^ 
the ecliptic is divided into twelve equal parts, called signs, 
which bear the names of the constellations through which this 
circle passes in the heavens. The degrees measured on the 
meridians from north to south, or south to north, are called 
degrees of latitude ; those measured from east to west on the 
equator, the echptic, or any of the lesser circles are called 
degrees of longitude ; hence these circles bear the name of 
longitudinal circles ; they are also called parallels of latitude. 

Emily, The degrees of longitude must then vary in length 
according to the dimensions of the circle on which they are 
reckoned ; those^ for instance, at the polar circles will be 
considerably smaller than those at the equator ? 

Mrs, B, Certainly ; since the degrees of circles of differ- 
ent dimensions do not vary in number, they must necessarily 
9# 



I0::i ON THE EARTB. 

vary in length. The degrees of latitude, 3^011 may observe^ 
never vary m length ; for the meridians on which they axe 
reckoned are all of the same dimensions. 

Emily. And of what length is a degree of latitude ? 

Mrs, B, Sixty geographical miles, which is equal to 69 i 
English statute miles. 

Emily, The degrees of longitude at the equator must then 
be of the same dimensions ? 

Mrs, B, They would, were the earth a perfect sphere ; 
but its form is not exactly* spherical, being somewhat protu- 
berant about the equator, and flattened towards the poles. 
This form is supposed to proceed from the superior action of 
the centrifugal power at the equator. 

Caroline, I thought I had understood the centrifugal force 
perfectly, but I do not comprehend its effect in this instance. 

Mrs. B, You know that the revolution of the earth on its 
axis must give every particle a tendency to fly ofl* from the 
centre, that this tendency is stronger or weaker in proportion 
to the velocity with which the y article moves ; now a parti- 
cle situated near one of the polar circles makes one rotation 
in the same space of time as a particle at the equator ; the 
latter, therefore, having a much larger circle to describe^ 
travels proportionally faster, consequently the centrifugal 
force is much stronger at the equator than at the polar circles : 
it gi'adually decreases as you leave the equator and approach 
the poles, where, as there is no rotatory motion, it entirely 
ceases. Supposing, therefore, the earth to have been origin- 
ally in a fluid state, the particles in the torrid zone would 
recede much farther from the centre than those in the frigid 
zones ; thus the polar regions would become flattened, and 
those about the equator elevated. 

Caroline. I did not consider that the particles in the 
neighborhood of the equator move with greater velocity than 
those about the poles ; this was the reason I could not under- 
stand you. 

Mrs, B. You must be careful to remember, that those 
j^arts of a body which are farthest from the centre of motion 
must move with the greatest velocity : the axis of the earth 
is the centre of its diurnal motion, and the equatorial regions 
the parts most distant from the axis. 

Caroline. My head then moves faster than my feet ; and 
upon the summit of a mountain we are carried round quicker 
^anin a vallev? 



Mrs* B. Certainlyj your head is more distant from the 
centre of motion, than your feet; the mountain-top than the 
valley : and the more distant any part of a body is from the 
centre of motion, the larger is the circle it will describe, and 
the greater therefore must be its velocity. 

Emily, I have been reflecting that if the earth is not a 
perfect circle..... 

Mrs, B. A sphere you mean, my dear ; a circle is a 
round line, every part of which is equally distant from the 
centre ; a sphere or globe is a round body^ the surface of 
which is every where equally distant from the centre. 

Emily. If, then, the earth is not a perfect sphere, but 
prominent at the equator, and depressed at the poles, would 
not a body weigh heavier at the equator than at the poles ? 
For the earth being thicker at the equator, the attraction of 
gravity perpendicularly downwards must be stronger. 

Mrs, B. Your reasoning has some plausibility, but I am 
sorry to be obliged to add that it is quite erroneous ; for the 
nearer any part of the surface of a body is to the centre of 
attraction, the more strongly it is attracted ; because the most 
considerable quantity of matter is about that centre. In re- 
gard to its effects, you might consider the power of gravity, as 
that of a magnet placed at the centre of attraction. 

Emily, But were you to penetrate deep into the earth, 
would gravity increase as you approached the centre ? 

Mrs, B, Certainly not ; I am referring only to any sit- 
uation on the surface of the earth. Were you to penetrate 
into the interior, the attraction of the parts above you would 
counteract that of the parts beneath you, and consequently 
diminish the power of gravity in proportion as you approached 
the centre ; and if you reached that point, being equally 
attracted by the parts all around you, gravity would cease, and 
you would be without weight. 

Emily, Bodies then should weigh less at the equator than 
at the poles, since they are more distant from the centre of 
gravity in the former than in the latter situation. 

Mrs^ B, And this is really the case ; but the difference 
of weight would be scarcely sensible, were it not augmented by 
another circumstance. 

Caroline, And what is this singular circumstance which 
seems to disturb the laws of nature ? 

Mrs. B, One that you are well acquainted with, as con- 
^^ucing more to the preservation than the destruction of order, 



104 ON THE EARTH, 

— ^the centrifugal force. This we have just observed to be 
stronger ai the equator ; and as it tends to drive bodies from 
the centre, it is necessarily opposed to, and must lessen the 
power of gravity? which attracts them towards the centre. 
We accordingly find that bodies weigh lightest at the equator, 
ivhere the centrifugal force is greaiest ; and heaviest at the 
poles, where this power is least.* 

Caroline. Has the experiment been made in these differ- 
ent situations ? 

Mrs. B, Lewis XIV., of France, sent philosophers both 
to the equator and to Lapland for this purpose ; the severity 
of the climate, and obstruction of the ice, has hitherto rendered 
every attempt to reach the pole abortive ; but the difference 
of gravity at the equator and in Lapland is very perceptible. 
Caroline. Yet I do not comprehend, how the difference 
of weight could be ascertained ; for if the body under trial 
decreased in weight, the weight which was opposed to it in 
the opposite scale must have diminished in the same propor- 
tion. For instance, if a pound of sugar did not weigh so heavy 
at the equator as at the poles, the leaden pound which served 
to weigh it, would not be so heavy either : therefore they 
would still balance each other, and the different force of 
gravity could not be ascertained by this means. 

Mrs. B. Your observation is perfectly just : the drfference 
of gravity of bodies situated at the poles and at the equator 
cannot be ascertained by weighing them ; a pendulum was 
therefore used for that purpose. 

Caroline. What, the pendulum of a clock ? how could 
that answer the purpose ? 

Mrs. B. A pendulum consists of a line, or rod, to one end 
of which a weight is attached, and it is suspended by ihe 
other to a fixed point, about which it is made to vibrate. 
Without being put in motion, a pendulum, like a plumb line, 
hangs perpendicular to the genera! surface of the earth, by 
which it is attracted ; but, if you raise a pendulum, gravity 
will bring it back to its perpendicular position. It will, how- 
ever, not remain stationary there, for the velocity it has 
received during its descent will impel it onwards, and it will 
rise on the opposite side to an equal height : from thence it 

* If the diurnal raotion of the earth round its axis was about 17 times 
faster than it is, th centrifugal force would, at the equator, be equal to 
the power of gravity, and all Hodi^s there would entirely lose weight 
Bat if the earth revolved still quicker thau this, they would all fly off. 



ON THE EARTH. 10? 

is brought back by gravity, and again driven by the impulse 
of its velocity. 

Caroline. If so, the motion of a pendulum would be per- 
petual, and I thought you said that there was no perpetual 
motion on the earth. 

Mrs, B, The motion of a pendulum is opposed by the 
resistance of the air in which it vibrates, and by the friction 
of the part by which it is suspended : were it possible to re- 
move these obstacles, the motion of a pendulum would be 
perpetual, and its vibrations perfectly regular : being of equal 
distances, and performed in equal times.* 

Emily, That is the natural result of the uniformity of the 
power which produces these vibrations, for the force of gravity 
being always the same, the velocity of the pendulum must 
consequently be uniform. 

Caroline. No, Emily, you are mistaken ; the cause is not 
always uniform, and therefore the effect will not be so either, 
I have discovered it, Mrs. B. ; since the force of gravity is 
less at the equator than at the poles, the vibrations of the 
pendulum will be slower at the equator than at the poles. 

Mrs, B, You are perfectly right, Caroline ; it was by this 
means that the difference of gravity was discovered, and the 
true figure of the earth ascertained. 

Emily, But how do they contrive to regulate their time in 
the equatorial and polar regions ? for, since in this part of the 
earth the pendulum of a clock vibrates exactly once in a 
second, if it vibrates faster at the poles and slower at the 
equator, the inhabitants must regulate their clocks in a differ- 
ent manner from ours. 

Mrs, B, The only alteration required is to lengthen the 
pendulum in one case, and to shorten it in the other ; for the 
velocity of the vibrations of a pendulum depends on its length ; 
and when it is said, that a pendulum vibrates quicker at the 
pole than at the equator, it is supposing it to be of the same 
length. A pendulum which vibrates a second in this latitude 

* The vibrations ot pendulums are subject to many irregularities, for 
which no effectual remedy has yet been devised. These are owing- partly 
to the variable density and temperature of the air, partly to the rigidity 
and friction of the rod by which they are suspended, and principally to 
the dilatation and contraction of the materials, of which they are formed. 
The metalline rods of pendulums are expanded by heat, and contracted 
by cold ; therefore clocks will go faster m winter^ and slower in summer. 
The common remedy for this inconvenience is the raising or lowering th© 
bob ©f the peadulum, by raieaDS of a screwj as occasion may re<jujre. 



106 ON THE EARTH. 

is 064 inches long. In order to vibrate at the equator in the 
same space of time, it must be lengthened by the addition 
of a few lines ; and at the poles, it must be proportionally 
shortened.* 

I shall now, I think, be able to explain to you the variation 
of the seasons, and the differefice of the lengih of the days and 
nights in those seasons ; both effects resulting from the same 
cause. 

In moving round the sun, the axis of ihe earth is not per- 
pendicular to the plane of its orbit. Supposhig tliis round 
table to represent the plane of the earth^'s orbit, and this litde 
globe, which has a wire passing through it, representing the 
axis and poles, we shall call the earth ; in moving round the 
table, the wire is not perpendicular to it, but oblique. 

Emily, Yes, I understand the earth does not go round the 
sun in an upriglit position, its axis is slanting or oblique. 

Mrs, S. All the lines, which you learnt in your last lesson, 
are delineated on this little globe ; you must consider the 
ecliptic as representing the plane of the earth's orbit ; and the 
equator which crosses the ecliptic in two places, shows the 
degree of obliquity of the axis of the earth in that orbit, which 
is exactly 234 degrees. The points in which the ecliptic 
intersects the equator are called nodes. 

Bui I believe I shall make this clear to you by revolving 
the little globe round a candle, w^hich shall represent the sun. 
(Plate IX: fig. 2.) 

As I now^ hold it, at A, you see it in the situation in which 
it is in the midsi of summer, or what is called the summer 
solstice, which is on the 21st of June. 

Emily, You hold the wire awry, I suppose, in order to 
show that the axis of the earth is not upright ? 

Mrs, B, Yes ; in summer, the north pole is inclined 
towards the sun. In this season, therefore, the northern 
hemisphere enjoys much more of his rays than the southern. 
The sun, you see, now shines over the whole of the north 

* What is here stated concerning the length of pendulums as connected 
with the force of gravity is at complete variance with fact. The force of 
gravitation is greater, it is well knowB, at the poles than at the equator ; 
and since the vibration of pendulums is occasioned by gravity, the lengths 
of pendulums vibrating in the same time must evidently be proportioned 
to the gravities at the places v/here they vibrate. Accoi-dingly, it is found^^ 
by observation, in order to vibrate, at the equator, in the same space, the 
pendulum must not be lengthened, as above stated, but shortened ; aiad, 
at the poles, it must not be shortened, but proportionally ieDgthened. 



ON THS EAETH, 107 

frigid zone^ and notwithstanding the earth's diurnal revolution, 
which I imitate by twirling the ball on the wire, it will con- 
tinue to shine upon it as long as it remains in this situation, 
whilst the south frigid zone is at the same thne completely in 
obscurity. 

Caroline. That is very strange : I never before heard 
that there was constant day or night in any part of the world ! 
How much happier the inhabitants of the north frigid zone 
must be than those of the southern ; the first enjoy uninter- 
rupted day, while the last are involved in perpetual darkness. 

Mrs, B. You judge with too much precipitation ; examine 
a little further, and you will find, that the two frigid zones 
share an equal fate. 

We shall now make the earth set off from its position in the 
summer solstice, and carry it round the sun ; observe th^t the 
pole is always inclined in the same direction, and points to the 
same spot in the heavens. There is a fixed star situated near 
that spot, which is hence called the North Polar star. Now 
let us stop the earth at B, and examine it in its present situa- 
tion ; it has gone through one quarter of its orbit, and is arrived 
at that point at which the ecliptic cuts or crosses the equator, 
and which is called the autumnal equinox. 

Emily. That is then one of the nodes. 

The sun now shines from one pole to the other, just as it 
would constantly do, if the axis of the earth were perpendicu- 
lar to its orbit. 

Mrs, B. Because the inclination of the axis is now neither 
towards the sun nor in the contrary direction ; at this period 
of the year, therefore, the days and nights are equal in every 
part of the earth. But the next step she takes in her orbit, 
you see, involves the north pole in darkness, whilst it illumines 
that of the south ; this change was gradually preparing as I 
moved the earth from summer to autumn ; the arctic circle, 
which was at first entirely illumined, began to have short 
nights J which increased as the earth approached the autumnal 
equinox ; and the instant it passed that point, the long night 
of the north pole commences, and the south pole begins to 
enjoy the light of the sun. We shall now make the earth 
proceed in its orbit, and you may observe that as it advances, 
the days shorten, and the nights lengthen, throughout the 
northern hemisphere, until it arrives at ^e winter solstice, on 
the 21st of December, when the north frigid zone is entirely 
in darkness, and the southern has uninterrupted day-light* 



108 ON THE EARTH. 

iJaroUne. Then after all, the sun which I thought so par- 
tial, confers his favors equally on all. 

Mrs. B. Not so neither : the inhabitants of the torrid 
zone have much more heat than we have, as the sun's rays 
fall perpendicularly on them, while they shine obliquely on 
the rest of the world, and almost horizontally on the poles ; 
for during their long day of six months, the sun moves round 
their horizon without either rising or setting ; the only obser- 
vable difference, is, that it is more elevated by a few degrees 
at mid-day, than at mid-night. 

Emily, To a person placed in the temperate zone, in the 
situation in which we are in England, the sun will shine 
neither so obliquely as it does on the poles, nor so vertically 
as at the equator ^ but its rays will fall upon him more 
obliquely in autumn and winter, than in summer. 

Caroline. And therefore, the inhabitants of the temperate 
zones, will not have merely one day and one night in the year 
as happens at the poles, nor will they have equal days and 
equal nights as at the equator ; but their days and nights will 
vary in length, at different times of the year, according as 
their respective poles incline towards or from the sun, and the 
difference will be greater in proportion to their distance from 
the equator. 

Mrs. B. We shall now follow the earth through the other 
lialf of her orbit, and you will observe, that now exactly the 
same effect takes place in the southern hemisphere, as what we 
have just remarked in the northern. Day commences at the 
south pole when night sets in at the north pole ; and in every 
other part of the southern hemisphere the days are longer 
than the nights, white, on the contrary, our nights are longer 
than our days. When the earth arrives at the vernal equinox, 
D, where the ecliptic again cuts the equator, on the 25th of 
March, she is situated with respect to the sun, exactly in the 
same position, as in tiie autumnal equinox ; and the only 
difference with respect to the earth, is, that it is now autumn 
in the southern hemisphere, whilst it is spring with us. 

Caroline. Then the days and nights are again every where 
equal ? 

Mrs. B. Yes, for the half of the globe which is enlightened, 
extends exactly from one pole to the other, the day breaks to 
the north pole, and the sun sets to the south pole; but in every 
other part of the globe, the day and night is of twelve hours 



0:S THE EAftTII. 109 

length J hence the word equinox^ which is derived from the 
Latin, meaning equal night. 

As the earth proceeds towards summer, the days lengthen 
in the northern hemisphere, and shorten in the southern, till 
the earth reaches the summer solstice, when the north frigid 
zone is entirely illumined, and the southern is in complete 
darkness ; and we have now brought the earth again to the 
spot from whence we first accompanied her. 

Emily, This is indeed; a most satisfactory explanation of 
the seasons ; and the more I learn, the more I admire the 
simplicity of means by which such wonderful effects are 
produced. 

Mrs, B, I know not which is most worthy of our admira- 
tion, the cause, or the effect of the earth's revolution, round 
the sun. The mind can find no object of contemplation, 
more sublime, than the course of this magnificent globe, 
impelled by the combined powers of projection and attraction 
to roll in one invariable course around the source of light and 
heat : and what can be more delightful than the beneficent 
effects of this vivifying power on its attendant planet. It is at 
once the grand principle which animates and fecundates 
nature. 

Emily. There is one circumstance in which this little 
ivory globe appears to me to differ from the earth ; it is not 
quite dark on that side of it, which is turned from the candle, 
as is the case with the earth when neither moon nor stars are 
visible. 

Mrs^ B. This is owmg to the light of the candle being 
reflected by the walls of the room on every part of the globe, 
consequently that side of the globe on which the candle does 
not directly shine, is not in total darkness. Now the skies 
have no walls to reflect the sun's light on that side of our 
earth which is in darkness. 

Caroline, I beg your pardon, Mrs. B., I think that the 
moon and stars answer the purpose of walls in reflecting the 
sun's light to us in the night. 

Mrs, B. Very well, Caroline ; that is to say, the moon 
and planets ; for the fixed stars, you know, shine by their 
own light. 

Emily, You say that the superior heat of the equatorial 
parts of the earth, arises from the rays falling perpendicularly 
on those regions, whilst they fall obliquely on these more 

io 



ilO ON THE EARTH. 

northern regions ; now I do not understand why perpendicu- 
lar rays should aflbrd more heat than oblique rays. 

Caroline. You need only hold your hand perpendicularly 
over the candle, and then hold it sideways oWiquely, to be 
sensible of the difference. 

Emily. I do not dowbt the fact, but I wish to have it 
explained. 

Mrs. B. You are quite right ; if Caroline had HOt been 
satisfied with ascertaining the fact, without understanding it, 
she would not have brought forward the candle as an illustra- 
tion ; the reason why you feel so much more heat if you hold 
your hand perpendicularly over the candle, than if you hold 
it sideways, is because a stream of heated vapour constantly 
ascends from the candle, or any other burning body, which 
being lighter than the air of the room, does not spread laterally 
but rises perpendicularly, and this led you to suppose that the 
rays were hotter in the latter direction. Had you reflected, 
you would have discovered that rays issuing from the candle 
sideways, are no less perpendicular to your hand w^hen held 
opposite to them, than the rays which ascend when your hand 
is held over them. 

The reason why the sun's rays afford less heat when in an 
oblique direction than when perpendicular, is because fewer 
of them fall upon an equal portion of the earth ; this will be 
understood better by referring to plate X. fig. 1, which repre- 
sents two equal portions of the sun's rays, shining upon differ- 
ent parts of the earth. Here it is evident that the same quan- 
tity of rays, fall on the space A B, as fall on the space B C ; 
and as A B is less than B C, the heat and light will be much 
stronger in the former than in the latter ; A B, you see, 
represents the equatorial regions, where the sun shines per- 
pendicularly ; and B C, the temperate and frozen climates, 
where his rays fall more obliquely. 

Emily. This accounts not only for the greater heat of the 
equatorial regions, but for the gi eater heat of summer ; as the 
sun shines less obliquely in summer than in winter* 

Mrs. B. This you will see exemplified in figure 2, in 
which the earth is represented, as it is situated on the 21st 
June, and England receives less oblique and consequently a 
greater number of rays, than at any other season ; and figure 
3, shows the situation of England on the 21st December, 
when the rays of the sun fall most obliquely upon her. But 
there is also another reason why oblique rays give less heat, 



PLATE A 





Tij 4- 



^■■^A 




ON THE EARTH* 111 

tiian perpendicular rays ; which is, that they have a greater 
portion of the atmosphere to traverse ; and though it is true, 
that the atmosphere is itself a transparent body, freely admit- 
ting the passage of the sun's rays, yet it is always loaded more 
or less with dense and foggy vapor, which the rays of the sun 
cannot easily penetrate ; therefore the greater the quantity of 
atmosphere the sun's rays have to pass through in their way 
to the earth, the less heat they will retain when they reach it. 
This will be better understood, by referring to figure 4. The 
dotted line round the earth, describes the extent of the atmos- 
phere, and the lines which proceed from the sun to the earth, 
the passage of two equal portions of the sun's rays to the 
equatorial and polar regions ; the latter, you see, from its 
greater obliquity passes through a greater extent of atmosphere. 

Caroline, And this, no doubt,^ is the reason why the sun 
in the morning and the evening gives so much less heat, than 
at mid-day. 

Mrs. B. The diminution of heat, morning and evening, is 
certainly owing to the greater obliquity of the sun's rays ; and 
as such they are affected by both the causes, which I have 
just explained to you ; the difficulty of passing through a 
foggy atmosphere is perhaps more particularly applicable to 
them, as mists and vapors are very prevalent about the time 
of sunrise and sunset. But the diminished obliquity of the 
sun's rays, is not the sole cause of the heat of summer ; the 
length of the da^^s greatly conduces to it : for the longer the 
sun is above the horizon, the more heat he will communicate 
to the earth. 

Caroline, Both the longest days, and the most perpen- 
dicular rays, are on the 21st of June ; and yet the greatest, 
heat prevails in July and August. 

Mrs, B, Those parts of the earth which are once heated, 
retain the heat for some length of time, and the additional 
heat they receive, occasions an elevation of temperature, 
although the days begin to shorten, and the sun's rays fall 
more obliquely. For the same reason, we have generally 
more heat at three o'clock in the afternoon, than at twelve 
when the sun is on the meridian. 

Emily, And pray, have the other planets the same vicis- 
situdes of seasons, as the earth ? 

Mrs, B, Some of them more, some less, according as their 
axes deviate more or less from the perpendicular to the plane 
of their orbits. The axis of Jupiter is nearly perpendicular 



112 ON THE £ARTH. 

to the plane of his orbit ; the axes of Mars and of Saturn are 
each inchned at angles of about sixty degrees ; whilst the axis 
of Venus is believed to be elevated only fifteen or twenty 
degrees above her orbit ; the vicissitudes of her seasons must 
therefore be considerably greater than ours. For further par- 
ticulars respecting the planets^ I shall refer you to Bonnycas- 
tle's Introduction to Astronomy. 

I have but one more observation to make to you relative to 
the earth's motion, which is, that although we have but 365 
days and nights in the year, she performs 366 complete 
revolutions on her axis during that time. 

Caroline* How is that possible ? for every complete 
revolution must bring the same place back to the sun. It is 
now just twelve o'clock, the sun is, therefore, on our meridian ; 
in twenty-four hours will it not he returned to our meridian 
again, and will not the earth have made a complete rotation 
on its axis. 

Mrs> B, If the earth had no progressive motion in its 
orbit whilst it revolves on its axis, this would be the case ; but 
as it advances almost a degree westward in its orbit, in the 
same time that it completes a revolution eastward on its axis, 
it must revolve nearly one degree more in order to bring the 
same meridian back to the sun. 

Caroline, Oh, yes ! it will require as much more of a 
second revolution to bring the same meridian back to the sun, 
as is equal to the space the earth has advanced in her orbit, 
that is, nearly a degree ; this difference is, however, very 
little. 

Mrs, B, These small daily portions of rotation are each 
equal to the three hundred and sixty-fifth part of a circle, 
which at the end of the year amounts to one complete rotation. 

Emily, That is extremely curious. If the earth, then, 
had no other than its diurnal motion, we should have S6G 
days in the year. 

Mrs, B, We should have 366 days in the same period of 
time that we now have S6^ ; but if we did not revolve round 
the sun, we should have no natural means of computing years, 

You will be surprised to hear, that if time is calculated by 
the stars instead of the sun, the irregularity which we have 
just noticed does not occur, and that one complete rotation of 
the earth on its axis, brings the same meridian back to any 
fixed star. 

fmihj. That seems quite unaccountable : for the eai'th 



ON THE EARTH. IW 

advances in her orbit with regard to the fixed stars, the same 
as with regard to the sun. 

Mrs, B, True J but then the distance of the fixed stars is 
so immensej that our solar system is in comparison to it but a 
spot, and the whole extent of the earth's orbit but a point ; 
therefore, whether the earth remained stationary, or whether 
it revolved in its orbit during its rotation on its axis, no sensible 
difference would be produced with regard to the fixed stars. 
One complete revolution brings the same meridian back to the 
same fixed star ; hence the fixed stars appear to go round the 
earth in a shorter time than the sun by three minutes fifty-six 
seconds of time. 

Caroline, These three minutes fifty-six seconds is the 
time which the earth takes to perform the additional three 
hundred and sixty-fifth part of the circle, in order to bring the 
same meridian back to the sun. 

Mrs, B, Precisely. Hence the stars gain every day 
three minutes fifty-six seconds on the sun, which makes them 
rise that portion of time earlier every day. 

When time is calculated by the stars it is called sidereal 
time, when by the sun solar or apparent time.* 

Caroline, Then a sidereal day is three minutes fifty-six 
seconds shorter than a solar day of twenty-four hours. 

Mrs, B, I must also explain to you what is meant by a 
sidereal year. 

The common year, called the solar or tropical year, con- 
taining 365 days, five hours, forty-eight minutes, and fifty-two 
seconds, is measured from the time the sun sets out from one 
of the equinoxes, or solstices, till it returns to the same again ; 
but this year is completed before the earth has finished one 
entire revolution in its orbit. 

Emily, I thought that the earth performed one complete 
revolution in its orbit every year ; what is the reason of this 
variation ? 

Mrs, B, It is owing to the spheroidal figure of the earth. 
The elevation about the equator produces much the same 

* If one clock should be so well regulated as to shew the time to be XII 
at noon this day, and on the 365th day afterward ; and another clock 
should be so well regulated as to show the time to be XII every day or 
night when any given star is on the meridian ; the latter clock would 
gain three minutes, fifty -five seconds, and fifty-four sixtieth parts of a 
second upon the former in each revolution of the same star to the met'i- 
dian. 

10* 



114 ON THE EARTH. 

eftect as if a similar mass of matter, collected in the form of a 
moon, revolved round the equator. When this moon acted 
on the earth in conjunction with or in opposition to the sun, 
variations in the earth's motion would be occasioned, and 
these variations produce what is called the precession of the 
equinoxes* 

Emily, What does that mean ? I thought the equinoctial 
points, or nodes, were fixeil points in the heavens, in which 
die equator cuts the ecliptic. 

Mrs, B. These points are not quite fixed, but have an 
apparently retrograde motion, that is to say, instead of being 
every revolution in the same place, they move backwards. 
Thus if the vernal equinox is at A, (fig. 1. plate XI.) the 
autumnal one will be at B instead of C, and the following 
vernal equinox at D instead of at A, as would be the case if 
the equinoxes were stationary at opposite points of the earth's 
orbit. 

Caroline. So that when the earth moves from one equi- 
nox to the other, though it takes half a year to perform th^ 
journey, it has not travelled through half its orbit. 

Mrs, B, And, consequently, when it returns again to the 
first equinox, it has not completed the whole of its orbit. In 
order to ascertain when the earth has performed an entire 
revolution in its orbit, we must observe when the sun returns 
in conjunction with any fixed star ; and this is called a side- 
real year. Supposing a fixed star situated at E, (fig. 1. plate 
XI.) the sun would not appear in conjunction with it till the 
earth had returned to A, when it would have completed its 
orbit. 

Emily, And how much longer is the sidereal than the 
solar year ? 

Mrs, B, Only twenty minutes ; so that the variation of 
the equinoctial points is very inconsiderable. I have given 
them a greater extent in the figure in order to render them 
sensible. 

In regard to time, I must further add, that the earth's diu¥- 
pal motion on an inclined axis, together with its annual rev- 
olution in an elliptic orbit, occasions so much comphcation in 
its motion, as to produce many irregularities ; therefore, true 
equal time cannot be measured by the sun. A clock, which 
was always perfectly correct, would in some parts of the year 
be before the sun, and in other parts after it. There are but 
ibur periods in which the sun and a perfect clock would' 




I. >;f 



ON THE EARTH. 115 

agree, which is the 15th of April, the l6th of June, the 23d 
of August, and the 24th of December. 

Emily, And is there any considerable difference between 
solar time and true time ? 

Mrs, B, The greatest difference amounts to between 
fifteen and sixteen minutes. Tables of equation are con- 
structed for the purpose of pointing out and correcting these 
differences between solar time and equal or mean time, which 
is the denomination giTen by astronomers to true time. 



CONVERSATION IX. 



ON THE MOON. 

Of the Moon^s Motion ; Phases of the Moon ; Eclipses of 
the Moon ; Eclipses of Jupiter's Moons ; Of the Lati- 
tude and Longitude ; Of the Transits of the Inferior 
Planets ; Of the Tides, 



MRS. B. 

We shall to-day confine our attention to the moon^ which 
offers many interesting phenomena. 

The moon revolves round the earth in the space of about 
twenty-nine days and a half, in an orbit nearly parallel to 
that of the earth, and accompanies us in our revolution round 
the sun. 

Emily. Her motion then must be rather of a complicated 
nature ; for as the earth is not stationary, but advances in 
her orbit whilst the moon goes round her, the moon must pro- 
ceed in a sort of progressive circle. 

Mrs, B, That is true ; and there are also other circum- 
stances which interfere with the simplicity and regularity of 
the moon's motion, but which are too intricate for you to 
understand at present. 

The moon always presents the same face to us, by which 
it is evident that she turns but once upon her axis, while she 
performs a revolution round the earth ; so that the inhabitants 
of the moon have but one day and one night in the course of a 
lunar month. 

Caroline. We afford them however the advantage of a 
magnificent moon to enlighten their long nights. 

Mrs. B* That advantage is but partial ; for since we al- 



ON THE MOON. Il7 

ways see the same hemisphere of the moorij the inhabitants 
of that hemisphere alone can perceive us. 

Caroline, One half of the moon then enjoys our light 
every night, while the other half has constantly nights of 
darkness. If there are any astronomers in those regions, 
they would doubtless be tempted to visit the other hemisphere, 
in order to behold so grand a luminary as we must appear to 
them. But, pray, do they see the earth under all the changes 
which the moon exhibits to us ? 

M7's, B, Exactly so. These changes are called the phases 
of the moon, and require some explanation. In figure 2, plate 
XI. let us say that S represents the sun, E the earth, and A 
BCD the moon in different parts of her orbit. When the 
moon is at A, her dark side being turned towards the earth, 
we shall not see her as at a ; but her disappearance is of very 
short duration, and as she advances in her orbit we perceive 
her under the form of a new moon ; when she has gone 
through one-eighth of her orbit at B, one quarter of her en- 
lightened hemisphere will be turned tow^ards the earth, and 
she will then appear horned as at b ; when she has performed 
one quarter of her orbit, she shows us one half of her enlight- 
ened side as at c ; at c? she is said to be gibbous, and at e the 
whole of the enlightened side appears to us, and the moon is at 
full. As she proceeds in her orbit she becomes again gib- 
bous, and her enlightened hemisphere turns gradually away 
from us until she completes her orbit and disappears, and 
then again resumes her form of a new moon. 

When the moon is at full, or a new moon, she is said to be 
in conjunction with the sun, as they are then both in the same 
direction w^ith regard to the earth ; when at her quarters she 
is said to be in opposition to the sun. 

Emily* Are not the eclipses produced by the moon pass- 
ing between the sun and the earth ? 

Mrs. B. Yes ; when the moon passes between the sun 
and the earth, she intercepts his rays, or in other words, casts 
a shadow on the earth, then the sun is eclipsed, and the day- 
light gives place to darkness, while the moon's shadow is 
passing over us. 

When, on the contrary, the earth is between the sun and 
the moon, it is we who intercept the sun's rays, and cast a 
shadow on the moon ; the moon is then darkened, she di-s 
appears from our view, and is eclipsed. 

Emily. But as the moon goes roynd the earth every 



118 ON THE MOON. 

month, she must be once during that time between the earth 
and the sun, and the earth must likewise be once between the 
sun and the moon, and yet we have not a solar and a lunar 
eclipse every month ? 

Mrs. B, The orbits of the earth and moon are not exactly 
parallel, but cross or intersect each other ; and the moon 
generally passes either above or below the earth when she is 
in conjunction with the sun, and does not therefore intercept 
the sun's rays, and produce an eclipse ; for this can take 
place only when the earth and moon are in conjunction in 
that part of their orbits which cross each other, (called the 
nodes of their orbits) because it is then only, that they are 
both in a right line with the sun. 

Emily, And a partial eclipse takes place, I suppose, when 
the moon in passing by the earth, is not sufficiently above or 
below the earth's shadow entirely to escape it ? 

Mrs. B. Yes, one edge of her disk then dips into the 
shadow, and is eclipsed ; but as the earth is larger than the 
moon, when the eclipse happens precisely at the nodes, they 
are not only total, but last for some length of time. 

When the sun is eclipsed, the total darkness is confined to 
one particular part of the earth, evidently showing that the 
moon is smaller than the earth, since she cannot entirely 
skreen it from the sun. In fig. 1. plate XII. you will find a 
solar eclipse described ; S is the sun, M the moon, and E the 
earth ; and the moon's shadow, you see, is not large enough 
to cover the earth. The lunar eclipses on the contrary are 
visible from every part of the earth, where the moon is above 
the horizon ; and we discover by the length of time which 
the moon is in passing through the earth's shadow, that it 
would be sufficient to eclipse her totally, were she 47 times 
her actual size ; it follows, therefore, that the earth is 47 times 
the size of the moon. 

In fig. 2. S represents the sun, which pours forth rays of 
light in straight lines in every direction. E is the earth, and 
M the moon. Now a ray of light coming from one extremity 
of the sun's disk in the direction A B, will meet another com- 
ing from the opposite extremity in the direction C B ; the 
shadow of the earth cannot therefore extend beyond B ; as 
the sun is larger than the earth, the shadow of the latter is 
conical, or the figure of a sugar loaf ; it gradually diminishes, 
and is much smaller than the earth where the moon passes 
through it, and yet we find the moon to be Bot only totally 



ON THE MOON. 119 

eclipsedj but some length of time in darkness, and hence we 
are enabled to ascertain its real dimensions. 

Emily, When the moon eclipses the sun to us, we must 
be eclipsed to the moon ? 

Mrs, B. Certainly ; for if the moon intercepts the sun^s 
raysj and cast a shadow on us, we must necessarily disappear 
to the moon, but only partially, as in fig. 1. 

Caroline. There must be a great number of eclipses in 
the distant planets, which have so many moons ? 

Mrs. jB. Yes, few days pass without an eclipse taking 
place ; for among the number of satellites, one or other of 
them are continually passing either between their planet and 
the sun, or between the planet and each other. Astrono- 
mers are so well acquainted with the motion of the planets 
and their satellites, that they have calculated not only the 
eclipses of our moon, but those of Jupiter, with such perfect 
accuracy, that it has afforded a means of ascertaining the 
longitude. 

Caroline, But is it not very easy to find both the latitude 
and longitude of any place by a map or globe ? 

Mrs, B, If you know where you are situated, there is no 
difiiculty in ascertaining the latitude or longitude of the place 
by referring to a map ; but supposing that you had been a 
length of time at sea, interrupted in your course by storms, a 
map would afford you very little assistance in discovering 
where you were. - 

Caroline, Under such circumstances, I confess I should 
be equally at a loss to discover either latitude or longitude. 

Mrs. B, The latitude may be easily found by taking the 
-altitude of the pole ; that it is to say the number of degrees 
that it is elevated above the horizon, for the pole appears 
more elevated as we approach it, and less as we recede 
from it. 

Caroline, But unless you can see the pole how can you 
take its altitude ? 

Mrs. B. The north pole points constantly towards one 
particular part of the heavens in which a star is situated, call- 
ed the Polar Star : this star is visible on clear nights, from 
every part of the northern hemisphere, the altitude of the 
polar star, is therefore the same number of degrees as that of 
the pole ; the latitude may also be determined by observa- 
tions made on the sun or any of the fixed stars ; the situation 
therefore of a vessel at sea, with regard to north and south, 



120 as THE MOON. 

is easily ascertained. The difficulty is respecting east and 
west, that is to say its longitude. As we have no eastern 
poles from which we can reckon our distance ; some particu- 
lar spot must be fixed upon for that purpose. The English 
reckon from the meridian of Greenwich, where the royal 
observatory is situated ; in French maps you will find that 
the longitude is reckoned from Paris. 

The rotation of the earth on its axis in 24 hours from 
west to east occasions, you know, an apparent motion of the 
sun and stars in the contrary direction, and the sun appears 
to go round the earth in the space of 24 hours, passing over 
fifteen degrees or a twenty-fourth part of the earth's circum- 
ference every hour ; therefore, when it is twelve o'clock in 
London, it is one o'clock in any place situated fifteen degrees 
to the east of London, as the sun must have passed the meri- 
dian of that place an hour before he reaches that of London. 
For the same reason it is eleven o'clock to any place situated 
fifteen degrees to the west of London, as the sun will not 
come to that meridian till an hour later. 

If then the captain of a vessel at sea, could know precisely 
what was thtB hour at London, he could, by looking at his 
watch, and comparing it w ith the hour of the spot in which he 
was, ascertain the longitude. 

Emily, But if he had not altered his watch, since he sail- 
ed from London, it would indicate the hour it was then in 
London. 

Mrs. B. True ; but in order to know the hour of the day 
of the spot in which he is, the captain of a vessel regulates his 
watch by th€ sun when it reaches the meridian. 

Emily, Then if he had two watches, he might keep one 
regulated daily, and leave the other unaltered ; the former 
would indicate the hour of the place in which he was situated, 
and the latter the hour of London ; and by comparing them 
together, he would be able to calculate his longitude. 

Mrs, B. You have discovered, Emily, a mode of finding 
the longitude, which I have the pleasure to tell you, is univer- 
sally adopted : watches of a superior construction, called 
chronometers, or time-keepers, are used for this purpose ; but 
the best watches are liable to imperfections, and should the 
time-keeper go too fast, or too slow, there would be no means 
of ascertaining the error ; implicit reliance cannot consequently 
be placed upon them. 

Recourse is therefore had to the eclipses of Jupiter's satel- 



ON THE MOON. 121 

lites. A table is made of the precise tinie at which the several 
moons are eclipsed to a spectator at London ; when they 
appear eclipsed to a spectator in any other spot, he may, by 
consulting the table, know what is the hour at London ; for 
the eclipse is visible at the same moment from whatever place 
on the earth it is seen. He has then only to look at the watch 
which points out the hour of the place in which he is, and by 
observing the difference of time there, and at London, he may 
immediately determine his longitude. 

Let us suppose, that a certain moon of Jupiter is always 
eclipsed at six o'clock in the evening ; and that a man at sea 
consults his watch, and finds that it is ten o'clock, at night, 
where he is situated, at the moment the eclipse takes place ; 
what will be his longitude ? 

Emily, That is four hours later than in London : four 
times fifteen degrees makes 60 ; he would, therefore, be sixty 
degrees east of London, for the sun must have passed his 
meridian before it reaches that of London. 

Mrs. B. For this reason the hour is always later than in 
London, when the place is east longitude, and earlier when it 
is west longitude. Thus the longitude can be ascertained 
whenever the eclipses of Jupiter's moons are visible. 

But it is not only the secondary planets which produce 
eclipses, for the primary planets near the sun eclipse him to 
those at a greater distance when they come in conjunction in 
the nodes of their orbits ; but as the primary planets are 
much longer in performing their course round the sun, than 
the satellites in going round their primary planets, these 
eclipses very seldom occur. Mercury and Venus have how- 
ever passed in a right line between us and the sun, but being 
at so great a distance from us, their shadows did not extend so 
far as the earth ; no darkness was therefore produced on any 
part of our globe ; but the planet appeared like a small black 
spot, passing across the sun's disk ; this is called a transit of 
the planet. 

It was by the last transit of Venus, that astronomers were 
enabled to calculate with some degree of accuracy the dis- 
tance of the earth from the sun, and the dimensions of the 
latter. 

Emily. I have heard that the tides are affected by the 
moon, but I cannot conceive what influence it can have on 
them. 

11 



122 ON THE MOON. 

Mrs. B, They are produced by the moon's attraction , 
which draws up the waters in a protuberance. 

Caroline, Does attraction act on water more powerfully 
than on land ? I should have thought it would have been just 
the contrary, for land is certainly a more dense body than 
water ? 

Mrs, B, Tides do not arise from water being more strong- 
ly attracted than land, for this certainly is not the case ; but 
the cohesion of jfluids being much less than that of solid bodies, 
they more easily yield to the power of gravity, in consequence 
of which the waters immediately below the moon are drawn 
up by it in a protuberance, producing a full tide, or what is 
commonly called high water, at the spot where it happens. 
So far the theory of the tides is not difficult to understand. 

Caroline. On the contrary, nothing can be more simple ; 
the waters, in order to rise up under the moon, must draw 
the waters from the opposite side of the globe, and occasion 
ebb-tide, or low water in those parts. 

Mrs. B. You draw your conclusion rather too hastily, my 
dear ; for according to your theory, we should have full tide 
only once in twenty-four hours, that is, every time that we 
were below the moon, while we find that we have two tides in 
the course of twenty-four hours, and that it is high water with 
us and with our antipodes at the same time. 

Caroline. Yet it must be impossible for the moon to at- 
tract the sea in opposite parts of the globe, and in opposite 
directions at the same time. 

Mrs. B. This opposite tide is rather more difficult to 
explain, than that which is drawn up beneath the moon ; 
with a little attention, however, I hope I shall be able to make 
you understand it. 

You recollect that the earth and moon are mutually attrac- 
ted towards a point, their common centre of gravity and of 
motion ; can you tell me what it is that prevents their meeting 
and uniting at this point ? 

Emily. Their projectile force, which gives them a tenden- 
cy to fly from this centre. 

Mrs. B. And is hence called their centrifugal force. 
Now we know that the centrifugal force increases in proportion 
to the distance from the centre of motion. 

Caroline. Yes, I recollect your explaining that to us, and 
illustrating it by the motion of the flyers of a wmd-mill, and 
the spinning of a top. 



ON THE MOUN. 123 

Emily, And it was but the other day you showed us that 
bodies weighed less at the equator, than in the polar regions, 
in consequence of the increased centrifugal force in the equa- 
torial parts. 

Mrs, B. Very well. The power of attraction, on the 
contrary, increases as the distance from the centre of gravity 
diminishes ; when, therefore, the two centres of gravity and 
of motion are in the same spot, as is the case with regard to 
the moon and the earth, the centrifugal power and those of 
attraction, will be in inverse proportion to each other ; that is 
to say, where the one is strongest, the other will be weakest. 

Emily, Those parts of the ocean, then, which are most 
strongly attracted will have least centrifugal force ; and those 
parts which are least attracted, will have the greatest centri- 
fugal force. 

Mrs, B, In order to render the question more simple, let 
us suppose the earth to be every where covered by the ocean, 
as represented in (fig. 3. pi. XII.) M is the moon, A B C D 
the earth, and X the common centre of gravity and of motion 
of these two planets. Now the waters on the surface of the 
earth, about A, being more strongly attracted than any other 
part, will be elevated ; the attraction of the moon at B and C 
being less, and at D least of all. But the centrifugal force at 
D, will be greatest, and the waters there, will in consequence 
have the greatest tendency to recede from the moon ; the 
waters at B and C will have less tendency to recede, and at 
A least of all. The waters, therefore, at D, will recede 
furthest, at the same time that they are least attracted, and in 
consequence will be elevated in a protuberance similar to that 
at A. 

Emily, The tide A, then, is produced by the moon's at- 
traction, and increased by the feebleness of the centrifugal 
power in those parts ; and the tide D is produced by the 
centrifugal force, and increased by the feebleness of the moon's 
attraction in those parts. 

Caroline, And when it is high water at A and D, it is 
low water at B and C : now I think I comprehend the nature 
of the tides again, though I confess it is not quite so easy as 
I at first thought. 

But, Mrs. B., why does not the sun produce tides as well 
as the moon ; for its attraction is greater than that of the 
moon ? 

Nrs. B, It would be at an equal distance, but our vicini- 



124 ON THE ^lOOX. 

ty to the moon makes her influence more powerful. The sun 
has, however, a considerable effect on the tides, and increases 
or diminishes them as it acts in conjunction with, or in oppo- 
sition to the moon. 

Etnily. I do not quite understand that. 

Mrs. B. The moon is a month in going round the earth ; 
twice during that time, therefore, at full and at change, she is 
in the same direction as the sun, both then act in conjufiction 
on the earth, and produce very great tides, called spring tides, 
as described in fig. 4. at A and B ; but when the moon is at 
the intermediate parts of her orbit, the sun, instead of afford- 
ing assistance, weakens her power by acting in opposition to 
it ; and smaller tides are produced, called neap tides, as rep- 
resented in fig. 5. 

Emihj. I have often observed the difference of these tides 
when I have been at the sea side. 

But since attraction is mutual between the moon and the 
earth, we must produce tides in the moon ; and these must 
be more considerable in proportion as our planet is larger. 
And 3^et the moon does not appear of an oval form. 

Mrs. B. You must recollect, that in order to render the 
explanation of the tides clearer, we suppose the whole sur- 
face of the earth to be covered with the ocean ; but that is 
not really the case, either with the earth or the moon, and 
the land which intersects the water destroys the regularity of 
the effect. 

Caroline, True ; we may, however, be certain, that 
wiienever it is high water the moon is immediately over our 
heads. 

Mrs. B. Not so either ; for as a similar effect is produc- 
ed on that part of the globe immediately beneath the moon, 
and on that part most distant from it, it cannot be over the 
heads of the inhabitants of both those situations at the same 
time. Besides, as the orbit of the moon is very nearly paral- 
lel to that of the earth, she is never vertical but to the inhab- 
itants of the torrid zone ; in that climate, therefore, the tides 
are greatest and they diminish as you recede from it and 
approach the poles. 

Caroline. In the torrid zone, then, I hope you will grant 
that the moon is immediately over, or opposite the spots 
where it is high water ? 

Mrs. B. I cannot even admit that ; for the ocean natur- 
ally partaking of the earth's motion, in its rotation from west 



ON THE MOO.V. 125 

fo east, the moon, in forming a tide, has to contend against 
the eastern motion of the waves. All matter, you know, 
by its inertia, makes some resistance to a change of state : 
the waters, therefore, do not readily yield to the attraction of 
the moon, and the effect of her influence is not complete till 
three hours after she has passed the meridian, where it is 
full tide. 

Emily, Pray what is the reason that the tide is three- 
quarters of an hour later every day ? 

Mrs. B. Because it is twenty-four hours and three-quar- 
ters before the same meridian on our globe returns beneath 
the moon. The earth revolves on its axis in about twenty- 
four hours ; if the moon were stationary, therefore, the same 
part of our globe would, every twenty-four hours, return 
beneath the moon ; but as during our daily revolution the 
moon advances in her orbit, the earth must make more than 
a complete rotation in order to bring the same meridian 
opposite the moon : we are three-quarters of an hour in 
overtaking her. The tides, therefore, are retarded for the 
same reason that the moon rises later by three-quai'ters of an 
hour every day. 

We have now, I think, concluded the observations I had 
to make to you on the subject of astronomy ; at our next 
interview, I shall attempt to explain to you the elements of 
hydrostatics. 



11^ 



CONVERSATION X. 



ON THE MECHANICAL PROPERTIES OF 
FLUIDS. 

Definition of a Fluid ; Distinction between Fluids and 
Liquids ; Of Non-Elastic Fluids ; Scarcely Susceptible 
of Cojupression ; Of the Cohesion of Fluids ; Of their 
Gravitation ; Of their Equilibrium ; Of their Pressure ; 
Of Specific Gravity ; Of the Specific Gravity of Bodies 
Heavier than Water ; Of those of the Same Weight as 
Water ; Of those Lighter than Water ; Of the Specific 
Gravity of Fluids. 



MRS. B. 

We have hitherto eonfined our attention to the mechanical 
properties of sohd bodies, which have been illustrated, and, I 
hope, thoroughly impressed upon your memory, by the con- 
versations we have subsequently had on astronomy. It will 
now be necessary for me to give you some account of the 
mechanical properties of fluids — a science which is called 
hydrostatics. A fluid is a substance which yields to the 
slightest pressure. If you dip your hand into a basin of water, 
you are scarcely sensible of meeting with any resistance. 

Emily, The attraction of cohesion is, then, I suppose, 
less powerful in fluids than in solids ? 

Mrs, B. Yes ; fluids, generally speaking, are bodies of 
less density than solids. From the slight cohesion of the 
particles of fluids, and the facility with which they slide over 
each other, it is inferred, that they must be small, smooth, and 
globular ; smooth, because there appears to be little or no 
friction among them : and globular, because touching each 



MECHANICAL PROPERTIES OF FLUID*. 12? 

Other but by a point would account for the slightness of their 
cohesion.* 

Caroline. Pray what is the distinction between a fluid 
and a Hquid ? 

Mrs, B* Liquids comprehend only one class of fluids. 
There is another class distinguished by the name of elastic 
fluids, or gases, which comprehends the air of the atmosphere, 
and all the various kinds of air with which you will become 
acquainted when you study chemistry. Their mechanical 
properties we shall examine at our next meeting, and confine 
our attention this morning to those of liquids, or non-elastic 
fluids. 

Water, and liquids in general, are scarcely susceptible of 
being compressed, or squeezed into a smaller space than that 
which they naturally occupy. This is supposed to be owing 
to the extreme minuteness of their particles, which, rather 
than submit to compression, force their way through the pores 
of the substance which confines them. This was show^n by a 
celebrated experiment, made at Florence many years ago. 
A hollow globe of gold was filled with water, and on its 
being submitted to great pressure, the water was seen to 
exude through the pores of the gold, which it covered with a 
fine dew. Fluids gravitate in a more perfect manner than 
solid bodies ; for the strong cohesive attraction of the parti- 
cles of the latter in some measure counteracts the efiects of 
gravity. In this table, for instance, the cohesion of the 
particles of wood enables four slender legs to support a 
considerable weight. Were the cohesion destroyed, or, in 
other words, the wood converted into a fluid, no support 
could be afforded by the legs, for the particles no longer 
cohering together, each would press separately and independ- 
ently, and would be brought to a level with the surface of the 
earth. 

Emily, This want of cohesion is then the reason why 
fluids can never be formed into figures, or maintained in 

* If the particles of fluids are round, there must be vacant spaces be- 
tween them, in the same manner as there are vacuities between cannon 
balls that are piled toj^ether ; between these balls smaller shot may be 
placed, and between these, others still smaller, or gravel, or sand, may be 
diffused. In a similar manner, a certain quantity of parfcles of sugar 
can be taken up in water without increasing its bulk, and when the water 
has dissolved the sugar, salt may be dissolved in it, and yet the bulk re- 
main the same ; arid admitting that the particles of water are round, this 
33 easily accounted for. 



128 MECHANICAL PROPERTIES OF FLUIDS. 

heaps ; for thougli it is true the wind raises water into waves^^ 
they are immediately afterwards destroyed by gravity, and 
water always finds its level. 

M7S, B, Do you understand what is meant by the level, 
or equilibrium of fluids ? 

Emily. I believe I do, though I feel rather at a loss to 
explain it. Is not a fluid level when its surface is smooth and 
flat, as is the case with all fluids when in a state of rest ? 

Mrs. B. Smooth, if you please, but not flat ; for the 
definition of the equilibrium of a fluid is, that every part of 
the surface is equally distant from the point to which gravity 
tends, that is to say, from the centre of the earth ; hence the 
surface of all fluids must be bulging, not flat, since they will 
partake of the spherical form of the globe. This is very evi- 
dent in large bodies of water, such as the ocean, but the 
sphericity of small bodies of water is so trifling, that their 
surfaces appear flat. 

This level, or equilibrium of fluids, is the natural result of 
dieir particles gravitating independently of each other ; for 
when any particle of a fl jid accidentally finds itself elevated 
above the rest, it is attracted down to the level of the surface 
of the fluid, and the readiness with which fluids yield to the 
slightest impression, will enable the particle by its weight to 
penetrate the surface of the fluid and mix with it. 

Caroline. But I have seen a drop of oil float on the sur- 
face of water without mixing with it. 

Mrs. B. That is, because oil is a lighter liquid than wa- 
ter. If you were to pour water over it, the oil would rise to 
the surface, being forced up by the superior gravity of the 
water. Here is an instrument called a water-level, (fig. 1. 
plate XIII.) which is constructed upon the principle of the 
equilibrium of fluids. It consists of a short tube, A B, 
closed at both ends, and containing a little water ; when the 
tube is not perfectly horizontal the water runs to the lower 
end, and it is by this means that the level of any situation to 
which we apply the instrument, is ascertained. 

Solid bodies you may, therefore, consider as gravitating in 
masses, for the strong cohesion of their particles makes them 
weigh altogether, while every particle of a fluid may be con- 
sidered as composing a separate mass, gravitating independ- 
ently of each other. Hence the resistance of a fluid is con- 
siderably less than that of a solid body ; for the resistance of 
the particles acting separately, they are more easily overcome = 



.MECHANICAL PROPERTIES OF FLUIDS. 129 

Emily. A body of water, in falling, does certainly less 
injury than a solid body of the same weight. 

Mrs, B. The particles of fluids acting thus independently, 
press against each other in every direction, not only down- 
wards but upwards, and laterally or sideways ; and in conse- 
quence of this equality of pressure, every particle remains at 
rest in the fluid. If you agitate the fluid you disturb this 
equahty of pressure and the fluid will not rest till its equili- 
brium is restored. 

Caroline, The pressure downwards is very natural ; it is 
the eflect of gravity, one particle weighing upon another pres- 
ses on it ; but the pressure sideways, and particularly the 
pressure upwards, I cannot understand. 

Mrs, B, If there were no lateral pressure, water would 
not run out of an opening on the side of a vessel. If you fill 
a vessel with sand, it will not run out of such an opening, 
because there is scarcely any lateral pressure among its 
particles. 

Emily, When water runs out of the side of a vessel, is it 
not owing to the w^eight of the water above the opening ? 

Mrs, B, If the particles of fluids were arranged in regular 
columns thus, (fig. 2.) there would be no lateral pressure, for 
when one particle is perpendicularly above the other, it can 
only press it downwards ; but as it must continually happen, 
that a particle presses between two particles beneath, (fig. 3.) 
these last must sufler a lateral pressure. 

Emily, The same as when a wedge is driven into a piece 
of wood, and separates the parts laterally. 

Mrs. B. Yes. Tlie lateral pressure proceeds, therefore, 
entirely from the pressure downwards, or the weight of the 
liquid above ; and consequently the lower the orifice is made 
in the vessel, the greater will be the velocity of the water 
rushing out of it. Here is a vessel of water (fig. 4.) with 
three stop cocks at different heights ; we shall open them, 
and you w^ill see with what different degrees of velocity the 
water issues from them. Do you understand this, Caroline?* 

Caroline, Oh yes. The water from the upper spout 
receiving but a slight pressure, on account of its vicinity to 

* An empty bottle Leing corked, and, by means of a weight, let down 
a certain depth into the sea, it wiil be broken, or the cork will be driven 
into it by the perpendicular pressure. But a bottle filled with water or 
any other liquid may be let down to any depth, without damage, because 
in this case the internal pressure is equal to the externa!. 



130 MECHANICAL PROPERTIES OP FLUIDS. 

the surface, flows but gently ; the second cock having a 
greater weight above it, the water is forced out with greater 
velocity, whilst the lowest cock being near the bottom of the 
vessel, receives the pressure of almost the whole body of water, 
and rushes out with the greatest impetuosity. 

Mrs, B» Very well ; and you must observe, that as the 
lateral pressure is entirely owing to the pressure downwards, 
it is not effected by the horizontal dimensions of the vessel, 
which contains the water, but merely by its depth ; for as 
every particle acts independently of the rest, it is only the 
column of particles, immediately above the orifice that can 
weigh upon and press out the water. 

Emily. The breadth and width of the vessel then can be 
of no consequence in this respect. The lateral pressure on 
one side, in a cubical vessel, is, I suppose, not so great as the 
pressure downwards. 

Mrs. B. No, in a cubical vessel the pressure downwards 
will be double the lateral pressure on vne side ; for every 
particle at the bottom of the vessel is pressed upon by a col- 
umn of the whole depth of the fluid, whilst the lateral pressure 
diminishes from the bottom upwards to the surface, where the 
particles have no pressure. 

Caroline. And from whence proceeds the pressure of 
fluids upwards ? that seems to me the most unaccountable, as 
it is in direct opposition to gravity. 

Mrs. B. And yet it is a consequence of their pressure 
downwards. When, for example, you pour water into a tea- 
pot, the w^ater rises in the spout to a level with the water in 
the pot. The particles of water at the bottom of the pot are 
pressed upon by the particles above them ; to this pressure 
they will yield, if there is any mode of making way for the 
superior particles, and as they cannot descend, they will change 
their direction and rise in the spout. 

Suppose the tea-pot to be filled with columns of particles 
of water similar to that described in fig. 4. the particle 1 at 
the bottom will be pressed laterally by the particle 2, and by 
this pressure be forced into the spout, where, meeting w^ith 
the particle 3, it presses n upwards and this pressure will be 
continued, from 3 to 4, from 4 to 5, and so on till the water 
in the spout has risen to a level with that in the pot. 

Emilf/. If it were not for this pressure upwards, forcing 
the water to rise in the spout, the equilibrium of the fluid would 
be destroved. 



MECHANICAL PROPERTIES OF FLUIDS. 131 

Caroline. True ; but then a tea-pot is wide and large^ 
and the weight of so great a body of water as the pot will con- 
tain, may easily force up and support so small a quantity as 
will fill the spout. But would the same effect be produced if 
the spout and the pot were of equal dimensions ? 

Mrs, B. Undoubtedly it would. You may even reverse 
the experiment by pouring water into the spout, and you will 
find that the water will rise in the pot to a level with that in 
the spout; for the pressure of the small quantity of water in the 
spout will force up and support the larger quantity in the pot. 
In the pressure upwards, as well as that laterally, you see 
that the force of pressure depends entirely on the height, and 
is quite independent of the horizontal dimensions of the fluid. 

As a tea-pot is not transparent, let us try the experiment 
by filling this large glass goblet by means of this narrow tube, 

(fig- 6.) 

Caroline, Lobit, Emily, as Mrs. B. fills it, how the water 
rises in the goblet, to maintain an equilibrium with that in the 
tube. 

Now, Mrs. B., will you let me fill the tube by pouring water 
into the goblet ? 

Mrs. B. That is impossible. However, you may try the 
experiment, and I doubt not but that you will be able to account 
for its failure. 

Caroline. It is very singular, that if so small a column of 
water as is contained in the tube can force up and support the 
whole contents of the goblet ; that the weight of all the water 
in the goblet should not be able to force up the small quantity 
required to fill the tube : — oh, I see now the reason, the water 
in the goblet cannot force that in the tube above its level, and 
as the end of the tube is considerably higher than the goblet, 
it can never be filled by pouring water into the goblet. 

Mrs. B. And if you continue to pour water into the gob- 
let when it is full, the water will run over instead of rising 
above the level in the tube. 

I shall now explain to you the meaning of the specific grav^ 
ity of bodies. 

Caroline. What ! is there another species of gravity with 
which we are not yet acquainted ? 

Mrs. B. No ; the specific gravity of a body, means sim- 
ply its weight compared with that of another body of the same 
size. When we say, that substances such as lead and stones 



132 MECHANICAL PROPERTIES OF FLUIDS. 

are heavy, and that others, such as paper and feathers, are 
light, we speak comparatively ; that is to say, that the first 
are heavy, and the latter light, in comparison with the general- 
ity of substances in nature. Would you call w^ood and chalk 
light or heavy bodies ? 

Caroline, Some kinds of wood are heavy certainly, as 
oak and mahogany ; others are light, as deal and box. 

Emily, I think I should call wood in general a heavy 
body, for deal and box are light only in comparison to wood 
of a heavier description. I am at a loss to determine whether 
chalk should be ranked as a heavy or alight body ; I should 
be inclined to say the former, if it was not that it is Hghter 
than most other minerals. I perceive that we have but vague 
notions of light and heavy. I wish there was some standard 
of comparison, to which we could refer the weight of all other 
bodies. 

Mrs, B, The necessity of such a standard has been so 
much felt, that a body has been fixed upon for this purpose. 
What substance do you think would be best calculated to 
answer this end ? 

Caroline, It must be one generally known and easily 
obtained, lead or iron for instance. 

Mrs, B, All the metals expand by heat, and condense by 
cold. A piece of lead, let us say a cubic inch for instance, 
w^ould have less specific gravity in summer than in winter ; 
for it would be more dense in the latter season. 

Caroline, But, Mrs. B., if you compare the weight of 
equal quantities of different bodies, they will all be alike. You 
know the old saying, that a pound of feathers is as heavy as a 
pound of lead. 

Mrs, B, When therefore we compare the weight of differ- 
ent kinds of bodies, it w^ould be absurd to take quantities of 
equal weight, we must take quantities of equal bulk ; pints or 
quarts, not ounces or pounds. 

Caroline, Very true ; I perplexed myself by thinking 
that quantity referred to weight, rather than to measure. It 
is true, it would be as absurd to compare bodies of the same 
size in order to ascertain which was largest, as to compare 
bodies of the same weight in order to discover which was 
heaviest. 

Mrs. B, In estimating the specific gravity of bodies, 
therefore, we must compare equal bulks, and we shall find 



MECHANICAL PROPERTIES OP FLUIDS. 133 

that their specific gravity will be proportional to their weights. 
The body which has been adopted as a standard of refeience 
is distilled water. 

Emily* I am surprised that a fluid should have been chos- 
en for this purpose, as it must necessarily be contained in 
some vesseij and the weight of the vessel will require to be 
deducted. 

Mrs, B, In order to learn the specific gravity of a solid 
body, it is not necessary to put a certain measure of it in one 
scale, and an equal measure of water into the other scale : 
but simply to weigh the body under trial in water. If you 
weigh a piece of gold in a glass of water, will not the gold 
displace just as much water, as is equal to its own bulk ? 

Caroline. Certainly, where one body is, another cannot 
be at the same time ; so that a sufficient quantity of water 
must be removed, in order to make way for the gold. 

Mrs, B. Yes, a cubic inch of water to make room for a 
cubic inch of gold ; remember that the bulk alone is to be 
considered, the weight has nothing to do with the quantity of 
water displaced, for an inch of gold does not occupy more 
space, and therefore will not displace more water than an 
inch of ivory, or any other substance, that will sink in water. 

Well, you will perhaps be surprised to hear that the gold 
will weigh less in water, than it did out of it. 

Emily. And for what reason ? 

Mr5. B. On account of the upward pressure of the par- 
ticles of water, which in some measure supports the gold, 
and by so doing, diminishes its weight. If the body immers- 
ed in water was of the same weight as that fluid, it w^ould be 
wholly supported by it, just as the water which it displaces 
was supported previous to its making way for the solid body. 
If the body is heavier than the water, it cannot be wholly 
supported by it ; but the water will offer some resistance to 
its descent. 

Caroline. And the resistance which water offers to the 
descent of heavy bodies immersed in it, (since it proceeds 
from the upward pressure of the particles of the fluid,) must 
in all cases, I suppose, be the same ? 

Mrs. B. Yes ; the resistance of the fluid is proportioned 
to the bulk, and not to the weight of the b dy immersed in it ; 
all bodies of the same size, therefore, lose the same quantity 
of their weight in water. Can you form any idea what this 
loss will be ? 

12 



134 MECHANICAL TROPERTIES OF FLUIDS. 

Emily, I should think it would be equal to the weight of 
the water displaced ; for, since that portion of the water was 
supported before the immersion of the solid body, an equal 
weight of the solid body will be supported. 

Mrs, B, You are perfectly right : a body weighed in 
water loses just as much of its weight, as is equal to that of 
the water it displaces ; so that if you were to put the water 
displaced into the scale to which the body is suspended, it 
would restore the balance. 

You must observe, that when you weigh a body in water, 
in order to ascertain its specific gravity, you must not sink the 
bason of the balance in the water ; but either suspend the 
body to a hook at the bottom of the bason, or else take off the 
bason, and suspend it to the arm of the balance, (fig. 7.) 
jNow suppose that a cubic inch of gold weighed 19 ounces 
out of water, and lost one ounce of its weight by being weigh- 
ed in water, what would be its specific gravity ? 

Caroline, The cubic inch of water it displaced must 
weigh that .one ounce ; and as a cubic inch of gold weighs 19 
ounces, gold is 19 times as heavy as water. 

Emily, I recollect having seen a table of the comparative 
weights of bodies, in which gold appeared to me to be estim- 
ated at 19 thousand times the weight of water. 

Mrs, B, You misunderstood the meaning of the table. 
In the estimation you allude to, the weight of water was reck- 
oned at 1000. You must observe, that the weight of a sub- 
stance when not compared to that of any other, is perfectly 
arbitrary ; and when water is adopted as a standard, we may 
denominate its weight by any number we please ; but then 
the weight of all bodies tried by this standard must be signifi- 
ed by proportional numbers. 

Caroline, We may call the weight of water, for example, 
one, and then that of gold would be nineteen ; or if we choose 
to call the weight of water 1000, that of gold would be 
19,000. In short, the specific gravity means how much more 
a body weighs than an equal bulk of water. 

Mrs, B, It is rather the weight of a body compared with 
that of water ; for the specific gravity of many substances is 
iess than that of water. 

Caroline, Then you cannot ascertain the specific gravity 
of such substances in the same manner as that of gold ; for a 
body that is lighter than water will float on its surface with- 
out displacing any water. 



MECHANICAL PROPERTIES OF FLUIDS. 135 

Mrs. B, If a body were absolutely light, it is true that it 
would not displace a drop of water, but the bodies we are 
treating of have all some weight, however small ; and will 
therefore, displace some quantity of water. If the body be 
lighter than water, it will not sink to a level with the surface 
of the water, and therefore it will not displace so much water 
as is equal to its bulk ; but it will displace as much as is equal 
to its weight. A ship, you must have observed, sinks to 
some depth in water, and the heavier it is laden the deeper it 
sinks, as it always displaces a quantity of water equal to its 
weight. 

Caroline, But you said just now, that in the immersion 
of gold, the bulk, and not the weight of body, was to be 
considered. 

Mrs. B. That is the case with all substances which are 
heavier than water ; but since those which are lighter do not 
displace so much as their own bulk, the quantity they displace 
is not a test of their specific gravit}^ 

In order to obtain the specific gravity of a body which is 
lighter than water, you must attach to it a heavy one, whose 
specific gravity is known, and immerse them together ; the 
specific gravity of the lighter body may then be easily cal* 
culated. 

Emily. But are there not some bodies which have exactly 
the same specific gravity as v/ater ? 

Mrs. B. Undoubtedly ; and such bodies will remain at 
rest in v/hatever situation they are placed in water. Here is 
a piece of wood which, by being impregnated with a little 
sand, is rendered precisely of the weight of an equal b ilk of 
water ; in whatever part of this vessel of water you place it, 
you will find that it will remain stationary. 

Caroline. I shall first put it at the bottom ; from thence, 
of course, it cannot rise, because it is not lighter than w^ater. 
Now I shall place it in the middle of the vessel ; it neither 
rises nor sinks, because it is neither lighter nor heavier 
than the water. Now I will lay it on the surface of the wa- 
ter ; but there it sinks a little— what is the reason of that. 
Mrs. B. ? 

Mrs. B. Since it is not lighter than the water, it cannot 
float upon its surface ; since it is not heavier than water, it 
cannot sink below its surface : it will sink therefore, only till 
the upper surface of both bodies are on a level, so that the 
piece of wood is just covered with water. If you poured a 



136 MECHANICAL PROPERTIES OP FLUIDS . 

few drops of water into the vessel, (so gently as not to increase 
their momentum by giving them velocity) they would mix 
with the water at the surface, and not sink lower. 

Caroline. This must, no doubt, be the reason why in 
drawing up a bucket of water out of a well, the bucket feels 
so much heavier when it rises above the surface of the water 
in the well ; for whilst you raise it in the water, the water 
within the bucket being of the same specific gravity as the 
water on the outside, will be wholly supported by the upward 
pressure of the water beneath the bucket, and consequently 
very little force will be required to raise it ; but as soon as 
the bucket rises to the surface of the well you immediately 
perceive the increase of weight. 

Emili/. And how do you ascertain the specific gravity of 
fluids ? 

Mrs. B, By means of an instrument called an hydrome- 
ter, which I will show you. It consists of a thin glass ball 
A, (fig. 8, plate XIII.) with a graduated tube B, and the 
specific gravity of the liquid is estimated by the depth to 
which the instrument sinks in it. There is a smaller ball, C, 
attached to the instrument below, which contains a little 
mercury ; but this is merely for the purpose of equipoising 
the instrument, that it may remain upright in the hquid un- 
der trial. 

I must now take leave of you ; bat there remain yet m any 
observations to be aiade on fluids ; we shall, therefore, re- 
sume this subject at our next interview. 



CONVERSATION XL 



OF SPRINGS, FOUNTAINS, &c. 

Of the Ascent of Vapor and the Formation of Clouds / 
Of the Formation and Fall of Rainy Sfc. ; Of the Form-* 
ation of Springs ; Of Pavers and Lakes ; Of Fountains^ 



CAROUNE. 

There is a question I am very desirous of asking you 
respecting fluids, JMrs. B., which has often perplexed me. 
What is the reason that the great quantity of rain which falls 
upon the earth and sinks into it, does not, in the course of 
time, injure its solidity ? The sun and the wind, I know, dry 
the surface, but they have no effect on the interior parts j 
where there must be a prodigious accumulation of moisture. 

Mrs, B, Do you not know that, in the course of time, all 
the v/ater which sinks into the ground rises out of it again ? 
It is the same water which successively forms seas, rivers, 
springs, clouds, rain, and sometimes hail, snow, and ice. If 
you will take the trouble of following it through these various 
changes, you will understand why the earth is not yet drown- 
ed by the quantity of water which has fallen upon it since its 
creation ; and you will even be convinced, that it does not 
contain a single drop more water now, than it did at that 
period. 

Let us consider how the clouds were originally formed. 
When the first rays of the sun warmed the surface of the 
earth, the heat, by separating the particles of v>^ater, rendered 
them lighter than the air. This, you know, is the case with 
steam or vapor. What then ensues ? 

Caroline. When lighter than the air it will naturally 
rise ; ^nd now I recollect vour telling us in a preceding les- 

12* 



138 OF SPRlN^Jb, FOUNTAINS . (IS^C. 

son, that the heat of the sun transformed the particles of 
water into vapor, in consequence of which it ascended into 
the atmosphere, where it formed clouds. 

Mrs. B. We have then already followed water through 
two of its transformations ; from water it becomes vapor, and 
from vapor clouds. 

Emihj, But since this watery vapor is lighter than the 
air, why does it not continue to rise ; and why does it unite 
again to form clouds. 

Mi^s. B. Because the atmosphere diminishes in density, 
as it is more distant from the earth. The vapor therefore 
which the sun causes to exhale, not only from seas, rivers, and 
lakes, but likewise from the moisture on the land, rises till it 
reaches a region of air of its own specific gravity ; and there, you 
know, it will remain stationary. By the frequent accession 
of fresh vapor it gradually accumulates, so as to form those 
large bodies of vapor, which we call clouds ; and these, at 
length, becoming too heavy for the air to support, they fall 
to the ground. 

Caroline, They do fall to the ground, certainly, when 
it rains ; but, according to your theory, I should have im- 
agined, that when the clouds became too heavy for the region 
of air in which they were situated to support them, they would 
descend till they reached a stratum of air of their own weight, 
and not fall to the earth ; for as clouds are formed of 
vapor, they cannot be so heavy as the lowest regions of the 
atmosphere, otherwise the vapor would not have risen. 

Mrs. B. If you examine the manner in which the clouds 
descend, it will obviate this objection. In falling, several of 
the watery particles come within the sphere of each other's 
attraction, and unite in the form of a drop of water. The 
vapor, thus transformed into a shower, is heavier than any 
part of the atmosphere, and consequently descends to the 
earth. 

Caroline. How wonderfully curious ! 

Mrs. B. It is impossible to consider any part of nature 
attentively without being struck with admiration at the wisdom 
it displays ; and I hope you will never contemplate these 
wonders without feeling your heart glow with admiration 
and gratitude towards their bounteous Author. Observe, 
that if the waters were never drawn out of the earth, all veg- 
etation would be destroyed by the excess of moisture ; if, on 
the other hand, the plants were not nourished and refreshed 



OP SPRINGS, FOUNTAlNSj SoC. ISP 

by occasional showers, the drought would be equally fatal to 
them. If the clouds constantly remain in a state of vapor, 
they might, as you remarked, descend into a heavier stratum 
of the atmosphere, but could never fall to the ground ; or 
were the power of attraction more than sufficient to convert 
the vapour into drops, it would transform the cloud into a 
mass of water, which, instead of nourishing, would destroy 
the produce of the earth. 

Water then ascends in the form of vapor, and descends in 
that of rain, snow, or hail, all of which ultimately become 
water. Some of this falls into the various bodies of water on 
the surface of the globe, the remainder upon the land. Of 
the latter, part re-ascends in the form of vapor, part is absorb- 
ed by the roots of vegetables, and part descends into the 
bowels of the earth, where it forms springs. 

Emili/, Is rain and spring-water then the same ? 

J\hs, B. Yes, originally. The only difference between 
rain and spring water, consists in the foreign particles which 
the latter meets with and dissolves in its passage through the 
various soils it traverses. 

Caroline. Yet spring water is more pleasant to the taste, 
appears more transparent, and, I should have supposed^ 
would have been more pure than rain water. 

Mrs. B, No ; excepting distilled water, rain water is 
the most pure we can obtain ; and it is its purity which ren- 
ders it insipid, whilst the various salts and different ingredi- 
ents, dissolved in spring water, give it a species of flavor, 
without in any degree affecting its transparency ; and the 
filtration it undergoes through gravel and sand in the bowels 
of the earth, cleanses it from all foreign matter which it has 
not the povv^er of dissolving. 

When rain falls on the surface of the earth, it continues 
m^aking its way downwards through the pores and crevices in 
the ground. When several drops meet in their subterrane- 
ous passage, they unite and form a little rivulet : this, in its 
progress, meets with other rivulets of a similar description 
and they pursue their course together in the bowels of the 
earth, till they are stopped by some substance which they 
cannot penetrate. 

Caroline, But you said that water could penetrate even 
the pores of gold, and they cannot meet with a substance 
more dense ? 

Mrs. B. But water penetrates the pores of gold only 



140 OF SPRINGS, FOUNTAINS, &C. 

when under a strong compressive force, as in the Florentine 
experiment ; now in its passage towards the centre of the 
earth, it is acted upon by no other power than gravit}^, which 
is not sufficient to make it force its w^ay even through a stra- 
tum of clay. This species of earth, though not remarkably 
dense, being of great tenacity, will not admit the particles of 
water to pass. When w^ater encounters any substance of 
this nature therefore, its progress is stopped, and the pressure 
of the accumulating waters forms a bed, or reservoir. This 
will be more clearly explained by fig. 9. plate XIII. which 
represents a section, or the interior of a hill or mountain. 
A, is a body of water such as I have described, whicJi, when 
filled up as high as B, (by the continual accession of water it 
receives from the ducts or rivulets a, «, a, «,) finds a passage 
out of the cavity, and, impelled by gravity, it runs on, till it 
makes its w^ay out of the ground at the side of the hill, and 
there forms a spring, C. 

Caroline. Gravity impels downwards towards the centre 
of the earth ; and the spring in this figiu'e runs in a horizon- 
tal direction. 

Mrs, B. Not entirely. There is some declivity from the 
reservoir to the spot w^iere the water issues out of the ground ; 
and gravity you know will bring bodies down an inclined 
plane, as well as in a perpendicular direction. 

Caroline, But though the spring may descend on first 
issuing, it must afterwards rise to reach the surface of the 
earth ; and that is in direct opposition to gravity. 

Mrs, B, A spring can never rise above the level of the 
reservoir whence it issues ; it must, therefore, find a passage 
to some part of the surface of the earth that is lower or nearer 
the centre than the reservoir. It is true that, in this figure, 
the spring rises in its passage from B to C occasionally ; but 
this, I think, with a little reflection, you will be able to ac- 
count for. 

Emily, Oh yes ; it is owing to the pressure of fluids up- 
wards, and the water rises in the duct upon the same princi- 
ple as it rises in the spout of a tea-pot ; that is to say, in order 
to preserve an equilibrium with the water in the reservoir. 
Now I think I understand the nature of springs ; the water 
will flow through a duct, whether ascending or descending, 
provided it never rises higher than the reservoir. 

Mas. B. Water may thus be conveyed to every part of a 
town, and to the upper part of the houses, if it is originally 



,Fu; J. 



TLAT£ XDT. 
Fiq. 2. 






J 






*;■:• 


B 


'■\^ 






OF SPRINGS, FOLNTAINSj&C. 141 

brought from a height superior to any to which it is convey- 
ed. Have you never observed, when the pavement of the 
streets liave been mending, the pipes which serve as ducts for 
the conveyance of the water through the town ? 

Emily, Yes, frequently ; and I have remarked that when 
any of these pipes have been opened, the water rushes up- 
wards from them with great velocity, which I suppose pro- 
ceeds from the pressure of the water in the reservoir, which 
forces it out. 

Caroline. I recollect having once seen a very curious 
glass, called Tantalus's cup ; it consists of a goblet, contain- 
ing a small figure of a man, and whatever quantity of water 
you pour into the goblet, it never rises higher than the breast 
of the figure. Do you know how that is contrived ? 

Mrs, B. It is by means of a syphon, or bent tube, which 
is concealed in the body of the figure. It rises through one 
of the legs as high as the breast, and there turning descends 
through the other leg, and from thence through the foot of 
the goblet where the water runs out. (fig. 1. plate XIV.) 
When you pour water into the glass A, it must rise in the 
syphon B, in proportion as it rises in the glass ; and when the 
glass is filled to a level with the upper part of the syphon, the 
water will run out through the other leg of the figure, and will 
continue running out, as fast as you pour it in ; therefore the 
glass can never fill any higher. 

Emilif, I think the new well that has been made at our 
country-house, must be of that nature. We had a great scar- 
city of water, and my father has been at considerable expense 
to dig a well ; after penetrating to a great depth before water 
could be found, a spring was at length discovered, but the 
water rose only a few feet above the bottom of the well ; and 
sometimes it is quite dry. 

Mrs. B. This has, however, no analogy to Tantalus's 
cup, but is owing to the very elevated situation of your 
country-house. 

Emily, I believe I guess the reason. There cannot be a 
reservoir of water near the summit of a hill ; as in such a situ- 
ation there will not be a sufficient number of rivulets formed 
to supply one ; and without a reservoir, there can be no 
spring. In such situations, therefore, it is necessary to dig^ 
very deep, in order to meet with a spring ; and when we 
give it vent, it can rise only as high as the reservoir from 



142 OF SPRINGS J FOUNTAINS, &G. 

whence it flows, which will be but little, as the reservoir must 
be situated at some considerable depth below the summit of 
the hill. 

Caroline. Your explanation appears very clear and satis- 
factory. But I can contradict it from experience. At the 
very top of a hill, near our country-house, there is a large 
pond, and, according to your theory, it would be impossible 
there should be springs in such a situation to supply it with 
water. Then you know that I have crossed the Alps, and I 
can assure you, that there is a fine lake on the summit of 
Mount Cenis, the highest mountain we passed over. 

Mrs, B. Were there a lake on the summit of Mount 
Blanc, which is the highest of the Alps, it would indeed be 
wonderful. But that on Mount Cenis, is not at ail contradic-* 
tory to our theory of springs ; for this mountain is surrounded 
by others much more elevated, and the springs which feed 
the lake must descend from reservoirs of water formed in 
those mountains. This must also be the case with the 
pond on the top of the hill : there is doubtless some more 
considerable hill in the neighborhood which supplies it with 
water. 

Emily, I comprehend perfectly, why the water in our 
well never rises high : but I do not understand why it should 
occasionally be dr3\ 

Mrs, B. Because the reservoir from which it flows being 
in an elevated situation, is but scantily supplied with water ; 
after a long drought, therefore, it may be drained, and the 
spring dry, till the reservoir be replenished by fresh rains. It 
is not uncornxUion to see springs flow with great violence in 
wet weather, and at other times be perfectly dry. 

Caroline, But there is a spring in our grounds which 
more frequently flows in dry than in wet weather : how is 
that to be accounted for ? 

Mrs, B, The spring probably comes from a reservoir 
at a great distance, and situated very deep in the ground : 
it is, therefore, some length of time before the rain reaches 
the reservoir, and another considerable portion must elapse, 
whilst the water is making its way from the reservoir to the 
surface of the earth ; so that the dry weather may probably 
have succeeded the rains before the spring begins to flow, and 
the reservoir may be exhausted by the time the wet weather 
sets in aG*ain. 



OF SPRINGS, FOUNTAINS, &;C, 143 

Caroline. I doubt not but this is the case, as the spring 
is in a very low situation, therefore the reservoir may be at a 
great distance from it. 

Mrs, B* Springs which do not constantly flow, are called 
intermitting, and are occasioned by the reservoir being im- 
perfectly supplied. Independently of the situation, this is 
always the case when the duct or ducts which convey the 
water into the reservoir are smaller than those which carry 
it off. 

Caroline . If it runs out faster than it runs in, it will of 
course sometimes be empty. And do not rivers also derive 
their source from springs ? 

Mrs. B. Yes, they generally take their source in moun- 
tainous countries, where springs are most abundant. 

Caroline. I understood you that springs were more rare 
in elevated situations. 

Mrs. B. You do not consider that mountainous countries 
abound equally with high and low situations. Reservoirs of 
water, which are formed in the bosom of mountains, generally 
find a vent either on their declivity, or in the valley beneath ; 
while subterraneous reservoirs formed in a plain, can seldom 
find a passage to the surface of the earth, but remain con- 
cealed, unless discovered by digging a well. When a spring 
once issues at the surface of the earth it continues its course 
externally, seeking always a lower ground, for it can no 
longer rise. 

Emily. Then what is the consequence, if the spring, or I 
should now rather call it a rivulet, runs into a situation, which 
is surrounded by higher ground. 

Mrs. B. Its course is stopped, the water accumulates, 
and it forms a pool, pond, or lake, according to the dimen- 
sions of the body of water. The lake of Geneva, in all 
probability, owes its origin to the Rhone, which passes through 
it : if, when this river first entered the valley, which now 
forms the bed of the Lake, it found itself surrounded by high- 
er grounds, its waters would there accumulate, till they rose 
to a level with that part of the valley where the Rhone now 
continues its course beyond the Lake, and from whence it 
flows through valleys, occasionally forming other small lakes 
till it reaches the sea. 

Emily. And are not fountains of the nature of springs ? 

Mrs. B. Exactly. A fountain is conducted perpendicu- 
larly upwards, by the spout or adjutage A, through which it 



144 OF SPRINGS, FOUNTAINS, &C. 

flows ; and it will rise nearly as high as the reservoir B, from 
whence it proceeds. (Plate XIV. figure 2.) 

Caroline, Why not quite as high ? 

Mrs. B, Because it meets with resistance from the air in 
its ascent ; and its motion is impeded by friction against the 
spoutj where it rushes out. 

Emily. But if the tube through which the water rises be 
smooth, can there be any friction ? especially with a fluid 
whose particles yield to the shghtest impression. 

Mrs. B. Friction, (as we observed in a former lesson,) 
may be diminished by polishing, but can never be entirely 
destroyed ; and though fluids are less susceptible of friction 
than solid bodies, they are still aflected by it. Another reason 
why a fountain will not rise so high as its reservoir, is, that 
as all the particles of water spout from the tube with an equal 
velocity, and as the pressure of the air upon the exterior 
particles must diminish their velocity, they will in some 
degree strike against the under parts, and force them sideways, 
spreading the column into a head, and rendering it both wider 
and shorter than it otherwise would be. 

At our next meeting, we shall examine the mechanical 
properties of the air, which being an elastic fluid, difters in 
many respects from liquids. 



CONVEHSATION XIL 



ON THE MECHANICAL PROPERTIES OF AIR. 

Of the Spring or Elasticity of the Air ; Of the weight of 
the Air ; Experiments icith the Air Pump ; Of the Ba- 
rometer ; Mode of weighing Air ; Specific Gravity of 
Air ; Of Pumps ; Description of the Sucking Pump ; 
Description of the Forcing Pump, 



MRS. B, 

At our last meeting we examined the properties of fluids 
in general, and more particularly of such fluids as are called 
liquids. 

There is another class of fluids, distinguished by the name 
of aeriform or elastic fluids, the principal of which is the air 
we breathe, which surrounds the earth, and is called the at- 
mosphere. 

Emily. There are then other kinds of air, besides the 
atmosphere ? 

Mrs, B, Yes ; a great variety ; but they differ only in 
their chemical, and not in their mechanical properties ; and 
as it is the latter we are to examine, we shall not at present 
inquire into their composition, but confine our attention to the 
mechanical properties of elastic fluids in general. 

Caroline, And from whence arises this difference ? 

Mrs, B, There is no attraction of cohesion between the 
particles of elastic fluids ; so tliat the expansive power of 
heat has no adversary to contend with but gravity ; any 
increase of temperature, therefore, expands elastic fluids 
prodigiously, and a diminution proportionally condenses 
them. 

13 



146 MECHANICAL PROPERTIES OF AIR. 

The most essential point in which air differs from other 
fluids, is by its spring or elasticity ; that is to say, its power 
of increasing or diminishing in bulk, according as it is more 
or less compressed : a power of which I have informed you 
liquids are almost wholly deprived. 

Emily. I think I understand the elasticity of the air very 
well from what you formerly said of it ; (see p. 33.) but what 
perplexes me is, its having gravity ; if it is heavy and we are 
surrounded by it, why do we not feel its weight ? 

Caroline. It must be impossible to be sensible of the 
weight of such infinitely small particles, as those of which the 
air is composed : particles which are too small to be seen, 
must be too light to be felt. 

Mrs. B. You are mistaken, my dear ; the air is much 
heavier than you imagine ; it is true, that the particles which 
compose it are small ; but then, reflect on their quantity : 
the atmosphere extends to about the distance of 45 miles from 
the earth ; and its gravity is such, that a man of middling 
stature is computed (when the air is heaviest) to sustain the 
weight of about 14 tons. 

Caroline. Is it possible ! I sliould have thought such a 
weight would have crushed any one to atoms. 

Mrs. B. That would, indeed, be the case, if it were not for 
the equality of the pressure on every part of the body ; but 
when thus diffused we can bear even a much greater weight, 
without any considerable inconvenience. In bathing we 
support the weight and pressure of the water, in addition to 
that of the atmosphere ; but because this pressure is equally 
distributed over the body, we are scarcely sensible of it ; 
whilst if your shoulders, your head, or any particular part of 
your frame were loaded with the additional weight of a hun- 
dred pounds you would soon sink under the fatigue. Be- 
sides this, our bodies contain air, the spring of which counter- 
balances the weight of the external air, and renders us less 
sensible of its pressure. 

Caroline. But if it were possible to relieve me from the 
weight of the atmosphere, should I not feel more light and 
agile ? 

Mrs. B, On the contrary, the air within you meeting 
with no external pressure to restrain its elasticity, would dis- 
tend your body, and at length bursting the parts which con- 
nned it, put a period to your existence. 



:^1ECHAN1CAL PROPERTrES OF AHl. 1^3 

pump. A few strokes of the handle totally excludes the air 
from the body of the pump, and fills it with water, which, 
having passed through both the valves, runs out at the spout. 

Cai^oline. I understand this perfectly. When the piston 
is elevated, the air and the water successively rise in the 
pump ; for the same reason as the mercury rises in the ba- 
rometer. 

Emily, I thought that water was drawn up into a pump, 
by suction, in the same manner as water may be sucked 
through a straw. 

Mi^s. B. It is so, into the body of the pump ; for the 
power of suction is no other than that of producing a vacuum 
over one part of the liquid, into which vacuum the liquid is 
forced, by the pressure of the atmosphere on another part. 
The action of sucking through a straw, consists in drawing in 
and confining the breath, so as to produce a vacuum in the 
mouth ; in consequence of which, the air within the straw 
rushes into the mouth, and is followed by the liquid, into 
which the lower end of the straw is immersed. The princi- 
ple, you see, is the same ; and the only difference consists in 
the mode of producing a vacuum. In suction, the muscular 
powers answer the purpose of the piston and valves. 

Emily, Water cannot, then, be raised by a pump above 
32 feet ; for the pressure of the atmosphere will not sustain a 
column of water above that height. 

Mrs, B, I beg your pardon. It is true that there must 
never be so great a distance as 32 feet from the level of the 
water in the well, to the valve in the piston, otherwise the 
water would not rise through that valve ; but when once the 
water has passed that opening, it is no longer the pressure of 
air on the reservoir which makes it ascend ; it is raised by 
lifting it up, as you would raise it in a bucket, of which the 
piston formed the bottom. This common pump is, therefore, 
called the sucking, or lifting-pump, as it is constructed on both 
these principles. There is another sort of pump, called the 
forcing-pump : it consists of a forcing power added to the 
sucking part of the pump. This additional power is exactly 
on the principle of the syringe : by raising the piston you 
draw the water into the pump, and by descending it you force 
the water out. 

Caroline, But the water must be forced out at the upper 
part of the pump ; and I cannot conceive how that can be 
hue by descending the piston. 



154 MECHANICAL PROPERTIES OF AIR. 

Mrs, B. Figure 5 plate XIV. will explain the difficulty. 
The large pipe A B represents the sucking part of the pump, 
which differs from the lifting-pump, only in its piston P being 
unfurnished with a valve, in consequence of which the water 
cannot rise above it. When, therefore, the piston descends, 
it shuts the valve Y and forces the water (which has no other 
vent) into the pipe D : this is likewise furnished with a valve 
V, which, opening outwards, admits the water, but prevents 
its return. 

The water is thus first raised in the pump, and then forced 
into the pipe, by the alternate ascending and descending mo- 
tion of the piston, after a few strokes of the handle to fill the 
pipe, from whence the water issues at the spout. 

It is now time to conclude our lesson. When next we meet, 
I shall give you some account of wind, and of sound, which 
will terminate our observations on elastic fluids. 

Caroline* And I shall run into the garden, to have the 
pleasure of pumping, now that I understand the construction 
of a pump. 

Mrs. -B. And, to-morrow I hope you will be able to tell 
me; whether it is a forcing or a common lifting pump. 



MECHANICAL PROPERTIES OF AIR. 149 

Mrs. B. Nothing more easy. I shall exhaust the air 
trom this little bottle by means of the air-pump : and having 
emptied the bottle of air, or, in other words, produced a va- 
cuum within it, I secure it by turning this screw adapted to 
its neck : we may now find the exact weight of this bottle, by 
putting it into one of the scales of a balance. It weighs you 
see just two ounces ; but when I turn the screw, so as to 
admit the air into the bottle, the scale which coYitains it pre- 
ponderates. 

Caroline, No doubt, the bottle filled with air^ is heavier 
than the bottle void of air ; and the additional weight requir- 
ed to bring the scales again to a balance, must be exactly that 
of the air which the bottle now contains. 

Mrs. B. That weight, you see, is almost two grains. 
The dimensions of this bottle are six cubic inches. Six cubic 
inches of air, therefore, at the temperature of this room, 
weighs nearly 2 grains. 

Caroline. Why do you observe the temperature of the 
room, in estimating the weight of the air. 

Mrs. B. Because heat rarifies air, and renders it lighter ; 
therefore the warmer the air is which you weigh, the lighter it 
will be. 

If you should now be desirous of knowing the specific 
gravity of this air, we need only fill the same bottle with 
water, and thus obtain the weight of an equal quantity of 
water — which you see is 1515 grains ; now by comparing 
the weight of water to that of air we find it to be in the 
proportion of about 800 to 1. 

I will show you another instance of the weight of the 
atmosphere, which I think will please you : you know what 
a barometer is ? 

Caroline. It is an instrument which indicates the state of 
the weather, by means of a tube of quicksilver ; but how, I 
cannot exactly say. 

Mrs. B. It is by showing the weight of the atmosphere. 
The barometer is an instrument extremely simple in its 
construction : in order that you may understand it, I will 
show you how it is made. I first fill a glass tube A B/(fig. 3. 
plate XIV.) about three feet in length, and open only at one 
end, with mercury ; then stopping the open end with m}^ 
finger, I immerse it in a cup C, containing a little mercury. 

Emily. Part of the mercury which was in the tube, I 
observe, runs down into the cup ; but why does not the whole 

13* 



1 jO MECHANICAL PROPERTIES OF AlS. 

of it subside in the cup, for it is contrary to the law of tht 
cquiUbrium of fluids, that the mercury in the tube should not 
descend to a level with that in the cup. 

Mrs. B. The mercury that has fallen from the tube into 
the cup, has left a vacant space in the upper part of the tube, 
to which the air cannot gain access ; this space is therefore a 
perfect vacuum ; and consequently the mercury in the tube is 
relieved from the pressure of the atmosphere, whilst that in the 
cup remains exposed to it. 

Caroline, Oh, now I understand it ; the pressure of the 
air on die mercury in the cup forces it to rise in the tube, 
where it sustains no pressure. 

Emily, Or rather supports the mercury in the tube, and 
prevents it from falling. 

Mra. B. That comes to the same thing ; for the power 
that can support mercury in a vacuum, would also make it 
ascend when it met with a vacuum. 

Thus you see, that the equilibrium of the mercury is des- 
troyed only to preserve the general equilibrium of fluids. 

Caroline, But this simple apparatus is, in appearance, 
very unlike a barometer. 

Mrs, B, It is all that is essential to a barometer. The 
tube and the cup or vase are fixed on a board, for the con- 
v^enicnce of suspending it ; the board is graduated for the 
purpose of ascertaining the height at which the mercury stands 
in the tube ; and the small moveable metal plate serves to 
show that height with greater accuracy. 

Emily, And at what height will the weight of the atmos- 
phere sustain the mercury ? 

Mrs, B, About 28 inches, as you will see by this baro- 
meter ; but it depends upon the weight of the atmosphere, 
'.vhich varies much according to the state of the weather. The 
oreater the pressure of the air on the mercury in the cup, the 
liigher it will ascend in the tube. Now can you tell me 
whether the air is heavier in wet or dry weather ? 

Caroline, Without a moment's reflection, the air must be 
iieaviest in wet weather. It is so depressing, emd makes one 
ieel so heavy ; while in fine weather, I feel as light as a 
feather, and as brisk as a bee. 

Mrs, B, Would it not have been better to have answered 
with a moment's reflection, Caroline ? It would have convin- 
•edyou, that the air must be heaviest in dry weather, for it is 
*hen. that the mercury is found to ri^e in the tube, and conse- 



MECHANICAL PROPERTIES OF AIR. 151 

quently the mercury in the cup must be most pressed by the 
air : and you know^ that we estimate the dryness and fairness 
of the weather, by the height of the mercury in the barometer. 

Cm^oline. Why then does the air feel so heavy in bad 
weather ? 

Mrs, B, Because it is less salubrious when impregnated 
with damp. The lungs under these circumstances do not 
play so freely, nor does the blood circulate so well : thus 
obstructions are frequently occasioned in the smaller vessels, 
from which arise colds, asthmas, agues, fevers, &c. 

Emily » Since the atmosphere diminishes in density in the 
upper regions, is not the air more rare upon a hill than in a 
plain ; and does the barometer indicate this diflerence ? 

Mrs, B, Certainly. The hills in this country are not suffi- 
ciently elevated to produce any very considerable effect on the 
barometer ; but this instrument is so exact in its indications, 
that it is used for the purpose of measuring the height of moun- 
tains, and of estimating the elevation of balloons. 

Emihj, And is no inconvenience experienced from the 
thinness of the air in such elevated situations ? 

Mrs, B, Oh, yes ; frequently. It is sometimes oppres- 
sive, from being insufficient for respiration ; and the expan- 
sion which takes place in the more dense air contained within 
the body is often painful : it occasions distension, and some- 
times causes the bursting of the smaller blood-vessels in the 
nose and ears. Besides, in such situations, you are more 
exposed both to heat and cold ; for though the atmosphere is 
itself transparent, its lower regions abound with vapors and 
exhalations from the earth, which float in it, and act in some 
degree as a covering, which preserves us equally from the 
intensity of the sun's rays, and from the severity of th^ cold. 

Caroline, Pray, Mrs. B., is not the thermometer con- 
structed on the same principles as the barometer ? 

Mrs, B, Not at all. The rise and fall of the fluid in the 
thermometer is occasioned by the expansive power of heat, 
and the condensation produced by cold ; the air has no access 
to it. An explanation of it would, therefore, be irrelevant to 
our present subject. 

Emihj, I have been reflecting, that since it is the weight 
cf the atmosphere which supports the mercury in the tube of a 
barometer, it would support a column of any other fluid in the 
^ame manner, 



152 MECHANICAL PROPERTIES OF AIR. 

Mrs, B. Certainly ; but as mercury is heavier than all 
other fluids, it will support a higher column of any other 
fluid ; for two fluids are in equilibrium, when their height 
varies inversely as their densities. We find the weight of 
the atmosphere is equal to sustaining a column of water, for 
instance, of no less than 32 feet above its level. 

Caroline. The weight of the atmosphere, is then, as 
great as that of a body of water the depth of 32 feet ? 

Mrs, B, Precisely ; for a column of air of the height of 
the atmosphere, is equal to a column of water of 32 feet, or 
one of mercury of 28 inches. 

The common pump is constructed on this principle. By 
the act of pumping, the pressure of the atmosphere is taken 
off the water, which, in consequence, rises. 

The body of a pump consists of a large tube or pipe, whose 
lower end is immersed in the water which it is designed to 
raise. A kind of stopper, called a piston, is fitted to this 
tube, and is made to slide up and down it by means of a me- 
tallic rod fastened to the centre of the piston. 

Emily, Is it not similar to the syringe, or squirt, with 
which you first draw in, and then force out water ? 

Mrs, B, It is ; but you know that we do not wish to 
force the water out of the pump at the same end of the pipe 
at which we draw it in. The intention of a pump is to raise 
water from a spring or well ; the pipe is therefore placed 
perpendicularly over the water which enters it at the lower 
extremity, and it issues at a horizontal spout towards the 
upper part of the pump. The pump, therefore, is rather a 
more complicated piece of machiner}^ than the syringe. 

Its various parts are delineated in this figure : (fig. 4. plate 
XIV.) A B is the pipe or body of the pump, P the piston, V 
a valve, or little door in the piston, which, opening upwards, 
admits the water to rise through it, but prevents its returning, 
and Y a similar valve in the body of the pump. 

When the pump is in a state of inaction, the two valves 
are closed by their own weight ; but when, by drawing down 
the handle of the pump, the piston ascends, it raises a column 
of air which rested upon it, and produces a vacuum between 
the piston and the lower valve Y, the air beneath this valve, 
which is immediately over the surface of the water, conse- 
quendy expands, and forces its way through it ; the water, 
then, relieved fyom the pressure of the air, ascends into the 



MECHANICAL PROPERTIES OF AIR* 14? 

Caroline. This weight of the atmosphere, then, which 
I was so apprehensive would crush me, is, in reahty, essen- 
tial to my preservation. 

Emily. I once saw a person cupped, and was told that 
the swelling of the part under the cup was produced by taking 
away from that part the pressure of the atmosphere ; but I 
could not understand how this pressure produced such an 
effect. 

Mrs, B. The air pump affords us the means of making a 
great variety of interesting experiments on the weight and 
pressure of the air : some of them you have already seen. 
Do you not recollect, that in a vacuum produced within the 
air pump, substances of various weights fell to the bottom in 
the same time ; why does not this happen in the atmos- 
phere ? 

Caroline, I remember you told us it was owing to the 
resistance which light bodies meet with from the air during 
their fall. 

Mrs. B. Or, in other words, to the support which they 
received from the air, and which prolonged the time of their 
fall. Now, if the air were destitute of weight, how could it 
support other bodies, or retard their fall ? 

i shall now show you some other experiments, which 
illustrate, in a striking manner, both the weight and elasticity 
of air. I shall tie a piece of bladder over this glass receiver, 
which, you will observe, is open both at the top as well as 
below. 

Caroline. Why do you wet the bladder first ? 
Mrs. B. It expands by wetting, and contracts in drying ; 
it is also more soft and pliable when wet, so that I can make 
it fit better, and when dry it will be tighter. We must hold 
it to the fire in order to dry ; but not too near lest it should 
burst by sudden contraction. Let us now fix it on the air- 
pump and exhaust the air from underneath it— you will not be 
alarmed if you hear a noise ? 

Emily, It was as loud as the report of a gun, and the 
bladder is burst ! Pray explain how the air is concerned in 
this experiment. 

Mrs. B. It is the effect of the weight of the atmosphere 
on the upper surface of the bladder, when t had taken away 
the air from the under surface ; so that there was no longer 
any re-action to counterbalance the pressure of the atmos- 
phere on the receiver. You observed how the bladder was 



148 MECHANICAL PROPERTIES OF AIR'. 

pressed inwards by the weight of the external air^ in propor- 
tion as I exhausted the receiver : and before a complete vacu- 
um was formed, the bladder, unable to sustain the violence of 
the pressure, burst with the explosion you have just heard. 

1 shall now show you an experiment, which proves the 
expansion of the air, contained within a body when it is re- 
lieved from the pressure of the external air. You would not 
imagine that there was any air contained within this shrivel- 
led apple, by its appearance ; but take notice of it when 
placed within a receiver, from which I shall exhaust the air. 

Caroline, How strange, it grows quite plump, and looks 
like a fresh-gathered apple. 

Mrs, B, But as soon as I let the air again into the receiver, 
the apple you see returns to its shrivelled state. When I 
took away the pressure of the atmosphere the air within the 
apple expanded and swelled it out ; but the instant the 
atmospherical air was restored, the expansion of the internal 
air was checked and repressed, and the apple shrunk to hs 
former dimensions. 

You may make a similar experiment with this little blad- 
der, which you see is perfectly flaccid and appears to contain 
no air : in this state I shall tie up the neck of the bladder, so 
that whatever air remains within it may not escape, and then 
place it under the receiver. Now observe, as I exhaust the 
receiver, how the bladder distends ; this proceeds from the 
great dilatation of the small quantity of air which v/as inclosed 
within the bladder when I tied it up ; but as soon as I let the 
air into the receiver, that which the bladder contains, con- 
denses and shrinks into its small compass within the folds of 
the bladder. 

Emily, These experiments are extremely amusing, and 
they afford clear proofs both of the weight and elasticity of 
the air ; but I should like to know exactly how much the air 
weighs. 

Mrs, B. A column of air reaching to the top of the at- 
mosphere, and whose base is a square inch, weighs 15lbs. 
when the air is heaviest ; therefore every square inch of our 
bodies sustains a weight of 15lbs. : and if you wish to know 
the weight of the whole of the atmosphere, you must reckon 
how many square inches there are on the surface of the globe, 
and multiply them by 15. 

Emili/, But are there no means of ascertaining the weighi, 
of a small quantity of air ? 



CONVERSATION XIIL 



ON WIND AND SOUND. 

Of Wind in General; Of the Trade Wind', Of the 
Periodical Trade Winds ; Of the Aerial Tides ; Of 
Sounds in General ; Of Sonorous Bodies ; Of Musical 
Sounds ; Of Concord or Harmony^ and Melody. 



MRS.B. 

Well, Caroline, have you ascertained what kind of pump 
you have in your garden ? 

Caroline. I think it must be merely a lifting-pump, be- 
cause no more force is required to raise the handle than is 
necessary to lift its weight ; and in a forcing-pump, by raising 
the handle, you force the water into the smaller pipe, and the 
resistance the water offers must require an exertion of strength 
to overcome it. 

Mrs. B. I make no doubt you are right ; for lifting 
pumps, being simple in their construction, are by far the most 
common. 

I have promised to day to give you some account of the 
nature of wind. Wind is nothing more than the motion of a 
stream or current of air, generally produced by a partial 
change of temperature in the atmosphere ; for when any one 
part is more heated than the rest, that part is rarefied ; the 
equilibrium is destroyed, and the air in consequence rises. 
When this happens, there necessarily follows a motion of the 
surrounding air towards that part, in order to restore it ; this 
spot, therefore, receives winds from every quarter. Those 
who live to the north of it experience a north wind ; those to 
the south, a south wind : — do you comprehend this ? 

Caroline. Perfectly. But what sort of weather must 



I'JV ON^WINUANP SOUND. 

those people have, who live on the spot where these winds 
meet and interfere ? 

Mrs, B. They have turbulent and boisterous weather, 
whirlwinds, hurricanes, rain, lightning, thunder, &c. This 
stormy weather occurs most frequently in the torrid zone, 
where the heat is greatest : the air being more rarefied there, 
than in any other part of the globe, is lighter, and consequent- 
ly ascends ; whilst the air about the polar regions is contin- 
ually flowing from the poles to restore the equilibrium. 

Caroline. This motion of the air would produce a regular 
and constant north wind to the inhabitants of the northern 
hemisphere ; and a south wind to those of the southern 
hemisphere, and continual storms at the equator, where these 
two adverse winds would meet. 

Mrs, B, These winds do not meet, for they each change 
their direction before they reach the equator. The sun, in 
moving over the equatorial regions from east to west, rarefies 
the air as it passes, and causes the denser eastern air to flow 
westwards, in order to restore the equilibrium ; thus producing 
ti regular east wind about the equator. 

Caroline, The air from the west, then, constantly goes to 
meet the sun, and repair the disturbance which his beams 
have produced in the equilibrium of the atmosphere. But I 
wonder how you will reconcile these various winds, Mrs. B. : 
you first led me to suppose there was a constant struggle be- 
tween opposite winds at the equator producing storm and 
tempest ; but now I hear of one regular invariable wind, 
which must naturally be attended by calm weather. 

Emily, I think I comprehend it : do not these winds 
from the north and south combine with the easterly wind 
about the equator, and form what are called the trade-winds ? 

J rs. B, Just so, my dear. The composition of the two 
w^inds north and east, produces a constant north-east wind ; 
and that of the two winds south and east, produces a regular 
south-east wind : these winds extend to about thirty degrees 
on each side of the equator, the regions further distant from it 
experiencing only their respective north and south winds. 

Caroline, But, Mrs. B., if the air is constantly flowing 
from the poles to the torrid zone, there must be a deficiency 
of air in the polar regions ? 

Mrs, B, The light air about the equator, which expands 
and rises into the upper regions of the atmosphere, ultimately 
flows from thence back to the poles, to restore the equilibri- 



w>N WIND AND SOUND. 157' 

um : if it were not for this resource^ the polar atmospheric 
regions would soon be exhausted by the stream of air, which, 
in the lower strata of the atmosphere, they are constantly 
sending towards the equator. 

Caroline, There is then a sort of circulation of air in the 
atmosphere ; the air in the lower strata flowing from the 
poles towards the equator, and in the upper strata, flowing 
back from the equator towards the poles. 

Mrs. B, Exactly. I can show you an example of this 
circulation on a small scale. The air of this room being more 
rarefied than the external air, a wind or current of air is pour- 
ing in from the crevices of the windows and doors, to restore 
the equilibrium ; but the light air with which the room is 
filled must find some vent, in order to make way for the 
heavy air which enters. If you set the door a-jar, and hold 
a candle near the upper part of it, you will find that the flame 
will be blown outwards, showing that there is a current of air 
flowing out from the upper part of the room. Now place 
the candle on the floor close by the door, and you will per- 
ceive, by the inclination of the flame, that there is also a cur- 
rent of air setting into the room. 

Caroline, It is just so ; the upper current is the warm 
light air, which h> driven out to make wa}^ for the stream of 
cold dense air which enters the room lower down. 

Emily, I have heard, Mrs, B., that the periodical winds 
are not so regular on land as at sea : what is the reason of 
that? 

Mrs, B, The land reflects into the atmosphere a much 
greater quantity of the sun's rays than the water ; tlierefore, 
that part of the atmosphere which is over the land, is more 
heated and rarefied than that which is over the sea : this occa- 
sions the wind to set in upon the land, as v/e find that it regu- 
larly does on the coast of Guinea, and other countries in the 
torrid zone. 

Emily, I have heard much of the violent tempests occa- 
sioned by the breaking up of the monsoons ; are not they 
^Iso regular trade-winds ? 

Mrs, B, They are called periodical trade-winds, as they 
change their course every half-year. This variation is pro- 
duced by the earth's annual course round the sun, when the 
north pole is inclined towards that luminary one half of the 
year, the south pole the other half. During the summer of 
the northern hemisphere, the countries of Arabia, Persia. 
14 



158 ON WIND AND SOUND. 

India, and China, are much heated, and reflect great quanti' 
ties of the sun's rays into the atmosphere, by which it becomes 
extremely rarefied, and the equilibrium consequently destroy- 
ed. In order to restore it, the air from the equatorial south- 
ern regions, where it is colder, (as well as from the colder 
northern parts,) must necessarily have a motion towards those 
parts. The current of air from the equatorial regions 
produces the trade-winds for the first six months, in all the 
seas between the heated continent of Asia, and the equator. 
The other six months, when it is summer in the southern 
hemisphere, the ocean and countries towards the southern 
tropic are most heated, and the air over those parts most 
rarefied : then the air about the equator alters its course, and 
flows exactly in an opposite direction. 

Caroline. This explanation of the monsoons is very 
curious ; but what does their breaking up mean ? 

Mrs, B, It is the name given by sailors to the shifting of 
the periodical winds ; they do not change their course sud- 
denly, but by degrees, as the sun moves from one hemisphere 
to the other : this change is usually attended by storms and 
hurricanes, very dangerous for shipping ; so that those seas 
are seldom navigated at the season of the equinox. 

Emily, I think I understand the winds in the torrid zone 
perfectly well ; but w^hat is it that occasions the great variety 
of winds which occur in the temperate zones ? for, according 
to your theory, there should be only north and south winds in 
those climates. 

Mrs. B, Since so large a portion of the atmosphere as is 
over the torrid zone is in continued agitation, these agitations 
in an elastic fluid, which yields to the slightest impression, 
must extend every w^ay to a great distance ; the air, therefore, 
in all climates, will suffer more or less perturbation, according 
to the situation of the country, the position of mountains, val- 
leys, and a variety of other causes : hence it is easy to conceive, 
that almost every climate must be liable to variable winds. 

On the sea-shore, there is almost always a gentle sea-breeze 
setting in on the land on a summer's evening, to restore the 
equilibrium which had been disturbed by reflections from the 
heated surface of the shore during the day ; and when night 
has cooled the land, and condensed the air, we generally find 
it, towards morning, flowing back towards the sea. 

Caroline. I have observed that the wind, which ever 
way it blows, almost always falls about sun-set. 



ON WIND AND SOUND. 159 

Mrs. B. Because the rarefaction of air in the particu- 
lar spot which produces the wind, diminishes as the sun de- 
chnesj and consequently the velocity of the wind abates. 

Emily, Since the air is a gravitating fluid, is it not affect- 
ed by the attraction of the moon and the sun, in the same 
manner as the waters ? 

Mrs. B. Undoubtedly ; but the aerial tides are as much 
greater than those of water, as the density of water exceeds 
that of air, which, as you may recollect, we found to be about 
800 to 1. 

Caroline. What a prodigious protuberance that must 
occasion ; How much the weight of such a column of air must 
raise the mercury in the barometer ! 

Emily.. As this enormous tide of air is drawn up and 
supported, as it were by the moon, its weight and pressure, I 
should suppose, would be rather diminished than increased ? 

Mrs. B. The weight of the atmosphere is nehher increas- 
ed nor diminished by the aerial tides. The moon's attraction 
augments the bulk as much as it diminishes the weight of the 
column of air ; these effects, therefore, counterbalancing each 
other, the aerial lldes do not affect the barometer. 

Caroline. I do not quite understand that. 

Mrs, B. Let us suppose that the additional bulk of air at 
high tide raises the barometer one inch ; and on the other 
hand, that the support which the moon's attraction affords the 
air diminishes its weight or pressure, so as to occasion the 
mercury to fall one inch ; under these circumstances the mer- 
cury must remain stationary. Thus you see, that we can 
never be sensible of aerial tides by the barometer, on account 
of the equahty of pressure of the atmosphere, whatever be its 
height. 

The existence of aerial tides is not, however, hypothetical ; 
it is proved by the effect they produce on the apparent posi- 
tion of the heavenly bodies ; but this I cannot explain to you, 
till you understand the properties of light. 

Emily. And when shall we learn them ? 

Mrs. B. I shall first explain to you the nature of sound, 
which is intimately connected with that of air ; and I think at 
our next meeting we may enter upon the subject of optics. 

We have now considered the effects produced by the wide 
and extended agitation of the air ; but there is another kind 
of agitation of which the air is susceptible — a sort of vibratory 



lOO ON WIND AND SOUND. 

trembling motion, which, striking on the drum of the ear 
produces sound. 

Caroline. Is not sound produced by solid bodies ? The 
voice of animals, the ringing of bells, musical instruments, are 
all solid bodies. I know of no sound but that of the wind 
which is produced by the air. 

Mrs. B. Sound, I assure you, results from a tremulous 
motion of the air ; and the sonorous bodies you enumerate,, 
are merely the instruments by which that peculiar species of 
motion is communicated to the air. 

Caroline. What ! when I ring this little bell, is it the air 
that sounds, and not the bell ? 

Mrs. B. Both the bell and the air are concerned in the 
production of sound. But sound, strictly speaking, is a per- 
ception excited in the mind by the motion of the air on the 
nerves of the ear ; the air, therefore, as well as the sonorous 
bodies which put it in motion, is only the cause of sound, the 
immediate effect is produced by the sense of hearing : for, 
without this sense, there would be no sound. 

Emily. I can with difficulty conceive that. A person 
born deaf, it is true, lias no idea ^£ sound, because he hears 
none ; yet that does not prevent the real existence of sound, 
as all those who are not deaf can testify. 

Mrs. B, I do not doubt the existence of sound to all those 
who possess the sense of hearing ; but it exists neither in the 
sonorous body nor in the air, but in the mind of the person 
whose ear is struck by the vibratory motion of the air, pro- 
duced by a sonorous body. 

To convince you that sound does not exist in sonorous 
bodies, but that air or some other vehicle is necessary to its 
production, endeavor to ring the little bell, after I have sus- 
pended it under a receiver in the air-pump, from which I 
shall exhaust the air 

Caroline. This is indeed very strange : though I agitate 
it so violently, it does not produce the least sound. 

Mrs. B. By exhausting the receiver, I have cut off the 
communication between the air and the bell ; the latter, 
therefore, cannot impart its motion to the air. 

Caroline. Are you sure that it is not the glass, whicli 
covers the bell, that prevents our hearing it ? 

Mrs. B. That you may easily ascertain by letting the air 
into the receiver, and then ringing the belL 



ON WIND AND SOUND. lOl 

Caroline. Very true : I can hear it now almost as loud as 
if the glass did not cover it ; and I can no longer doubt but 
that air is necessary to the production of sound. 

Mrs, B. Not absolutely necessary, though by far the most 
common vehicle of sound. Liquids, as well as air are capa- 
ble of conveying the vibratory motion of a sonorous body to 
the organ of hearing ; as sound can be heard under water. 
Solid bodies also convey sound, as I can soon convince you 
by a very simple experiment. I shall fasten this string by 
the middle round the poker ; now raise the poker from the 
ground by the two ends of the string and hold one to each of 
your ears : — I shall now strike the poker with a key, and you 
will find that the sound is conveyed to the ear by means of the 
strings, in a much more perfect manner than if it had no other 
vehicle than the air. 

Caroline. That it is, certainly, for I am almost stunned by 
the noise. But what is a sonorous body, Mrs. B. ? for all 
bodies are capable of producing some kind of sound by the 
motion they communicate to the air. 

Mrs. B. Those bodies are called sonorous, which produce 
clear, distinct, regular and durable sounds, such as a bell, a 
drum, musical strings, wind-instruments, &c. They owe this 
property to their elasticity ; for an elastic body, after having 
been struck, not only returns to its former situation, but hav- 
ing acquired momentum by its velocity, like the pendulum, it 
springs out on the opposite side. If I draw the string A B, 
which is made fast at both ends to C, it will not only return 
to its original position, but proceed onwards to D. 

This is its first vibration, at the end of which it will retain 
sufficient velocity to bring it to E, and back again to F which 
constitutes its second vibration ; the third vibration will carry 
it only to G and H, and so on till the resistance of the air 
destroys its motion. 

The vibration of a sonorous body gives a tremulous motion 
to the air around it, very similar to the motion communicated 
to smooth water when a stone is thrown into it. This first 
produces a small circular wave around the spot in which the 
stone falls ; the wave spreads, and gradually communicates 
its motion to the adjacent waters, producing similar waves to 
a considerable extent. The same kind of waves are produced 
in the air by the motion of a sonorous body, but with this , 
difference, that as air is an elastic fluid, the motion does not 
consist of regularly extending waves, but of vibrations, and 
14* 



i 62 On wind and sound. 

are composed of a motion forwards and backwards, similar to 
those of the sonorous body. They differ also in the one 
taking place in a plane, the other in all directions. The 
aerial undulations being spherical. 

Emily, But if the air moves backwards as well as for- 
wards, how can its motion extend so as to convey sound to a 
distance. 

Mrs. B. The first sphere of undulations which are pro- 
duced immediately around the sonorous body, by pressing 
against the contiguous air, condenses it. The condensed air, 
though impelled forward by the pressure, re-acts on the first 
set of undulations, driving them back again. The second set 
of undulations v/hich have been put in motion, in their turn 
communicate their motion, and are themselves driven back by 
te-action. Thus there is a succession of waves in the air, 
corresponding with the succession of waves in the water. 

Caroline. The vibrations of sound must extend much 
further than the circular weaves in water, since sound is con- 
veyed to a great distance. 

Mrs. B. The air is a fluid so much less dense than water, 
that motion is more easily communicated to it. The report 
of a cannon produces vibrations of the air which extend to 
several miles around. 

Emily. Distant sound takes some time to reach us, since 
it is produced at the moment the cannon is fired ; and we see 
the light of the flash long before we hear the report. 

Mrs. B. The air is immediately put in motion by the 
firing of a cannon ; but it requires time for the vibrations to 
extend to any distant spot. The velocity of sound is computed 
to be at the rate of 1142 feet in a second. 

Caroline. With what astonishing rapidity the vibrations 
must be communicated ! But the velocity of sound varies, I 
suppose, with that of the air which conveys it. If the wind 
sets towards us from the cannon, we must hear the report 
sooner than if it set the other way. 

Mrs. B. The direction of the wind makes less difference 
in the velocity of sound than you would imagine. If the wind 
sets from us, it bears most of the aerial waves away and ren- 
ders the sound fainter ; but it is not very considerably longer 
in reaching the ear than if the wind blew towards us. This 
Tmiform velocity of sound enables us to determine the distance 
of the object from which it proceeds ; as that of a vessel at 
sea firing a cannon; or that of a thunder cloud. If we do not 



ON WIND AND SOUND. l6S 

hear the thunder till half a minute after we see the lightning, 
we conclude the cloud to be at the distance of six miles and a 
half. 

Emily. Pray how is the sound of an echo produced ? 

Mrs, B. When the aerial vibrations meet with an obsta- 
cle, having a hard and regular surface^ such as a wall, or rock^ 
they are reflected back to the ear, and produce the same sound 
a second time ; but the sound will then appear to proceed 
from the object by which it is reflected. If the vibrations fall 
perpendicularly on the obstacle, they are reflected back in the 
same line ; if obliquely, the sound returns obliquely in the 
opposite direction, the angle of reflection being equal to the 
angle of incidence. 

Caroline, Oh, then, Emily, I now understand why the 
echo of my voice behind our house is heard so much plainer 
by you than it is by me, when we stand at the opposite ends 
of the gravel walk. My voice, or rather, I should say, the 
vibrations of air it occasions, fall obliquely on the wall of the 
house, and are reflected by it to the opposite end of the gravel 
walk. 

Emily, Very true ; and we have observed that when we 
stand in the middle of the walk, opposite the house, the echo 
returns to the person who spoke. 

Mrs, B, Speaking-trumpets are constructed on the princi- 
ple of the reflection of sound. The voice, instead of being 
diflused in the open air, is confined within the trumpet ; and 
the vibrations which spread and fall against the sides of the 
instrument, are reflected according to the angle of incidence, 
and fall into the direction of the vibrations which proceed 
straight forwards. The whole of the vibrations are thus 
collected into a focus ; and if the ear be situated in or near that 
spot, the sound is prodigiously increased. Figure 7. plate 
XIV. will give you a clearer idea of the speaking-trumpet : 
the reflected rays are distinguished from those of incidence, 
by being dotted ; and they are brought to a focus at F. The 
trumpet used by deaf persons acts on the same principle ; 
but as the voice enters the trumpet at the large instead of the 
small end of the instrument, it is not so much confined, nor 
the sound so much increased. 

Emily, Are the trumpets used as musical instruments 
also constructed on this principle ? 

Mrs, B, So far as their form tends to increase the sound, 
they are ; but, as a musical instrument, the trumpet becomes 



164 ON WIND AND SOUND. 

itself the sonorous body, which is made to vibrate by blowing 
into itj and communicates its vibrations to the air. 

I will attempt to give you in a few words, some notion of the 
nature of musical sounds, which as you are fond of music must 
be interesting to you. 

If a sonorous body be struck in such a manner, that its 
vibrations are all performed in regular tim.es, the vibrations 
of the air will correspond with them ; and striking in the 
same regular manner on the drum of the ear, will produce the 
same uniform sensation on the auditory nerve and excite the 
same uniform idea in the mind ; or, in other words, we shall 
hear one musical tone. 

But if the vibrations of the sonorous body are irregular, 
there will necessarily follow a confusion of aerial vibrations ; 
for a second vibration may commence before the first is 
finished, meet it half way on its return, interrupt it in its 
course, and produce harsh jarring sounds which are called 
discords, 

Emily. But each set of these irregular vibrations, if repeat- 
ed at equal intervals, would, I suppose, produce a musical 
tone ? It is only their irregular succession which makes them 
interfere, and occasions discord. 

Mrs, B, Certainly. The quicker a sonorous body vibrates, 
the more acute^ or sharp, is the sound produced. 

Caroline, But if I strike any one note of the piano-forte 
repeatedly, whether quickly or slowly, it always gives the 
same tone. 

Mrs, B, Because the vibrations of the same string, at the 
same degree of tension, are always of a similar duration. The 
quickness or slowness of the "vibrations relate to the single 
tones, not to the various sounds which they may compose by 
succeeding each other. Striking the note in quick succession, 
produces a more frequent repetition of the tone, but does not 
increase the velocity of the vibrations of the string. 

The duration of the vibrations of strings or chords depends 
upon their length, their thickness or weight, and their degree 
of tension : thus, you find, the low bass notes are produced 
by long, thick, loose strings ; and the high treble notes by 
short, small, and tight strings. 

Caroline, Then the different length and size of the strings 
of musical instruments, serves to vary the duration of the 
vibrations, and consequently, the acuteness of gravity of the 
notes ? 



ON WIND AND SOUND. l65 

Mrs, B. Yes. Among the variety of tones, there are 
some whichj sounded together, please the ear, producing what 
we call harmony, or concord. This arises from the agree- 
ment of the vibrations of the two sonorous bodies ; so that 
some of the vibrations of each strike upon the ear at the same 
time. Thus, if the vibrations of two strings are performed in 
equal times, the same tone is produced by both, and they are 
said to be in unison. 

Emily, Now, then, I understand why, when I tune my 
harp in unison with the piano-forte, I draw the strings tighter 
if it is too low, or loosen them if it is at too high a pitch ; it is in 
order to bring them to vibrate, in equal times, with the strings 
of the piano-forte. 

Mrs, B, But concord, you know, is not confined to 
unison ; for two different tones harmonize in a variety of 
cases. If the vibrations of one string (or sonorous body 
whatever) vibrate in double the time of another, the second 
vibration of the latter will strike upon the ear at the same 
instant as the first vibration of the former 5 and this is the 
concord of an octave. 

If the vibrations of two Strings are as two to three, the 
second vibration of the first corresponds with the third viura^ 
tion of the latter, producing the harmony called a fifth. 

Caroline, So, then, when I strike the key-note with its 
fifth, I hear every second vibration of one, and every third of 
the other at the same time ? 

Mrs, B, Yes ; and the key-note struck with the fourth is 
likewise a concord, because the vibrations are as three to 
four. The vibrations of a major third with the key-note, are 
as four to five ; and those of a minor third, as five to six. 

There are other tones which, though they cannot be struck 
together without producing discord, if struck successively, 
gives us the pleasure which is called melody. Upon these 
general principles the science of music is founded ; but I am 
not sufficiently acquainted with it to enter any further into it. 

We shall now, therefore, take leave of the subject of 
sound ; and, at our next interview, enter upon that of optics, 
in which we shall consider the nature of vision, light, an^ 
colors. 



CONVERSATION XIV. 



ON OPTICS. 

Of Luminous J Transparent^ and Opaque Bodies : Of the 
Radiation of Light ; Of Shadows ; Of the Reflection of 
Light ; Opaque Bodies seen only hy Reflected Light ; 
Fision explained ; Camera Obscura ; Image of Objects 
on the Retina, 



CAROUNE. 

I LONG to begin our lesson to-day, Mrs, B., for I expect 
that it will be ver}^ entertaining. 

Mrs, B. Optics is certainly one of the most interesting 
branches of Natural Philosophy, but not one of the easiest to 
understand ; I must therefore beg that you will give me the 
whole of your attention. 

I shall first inquire, whether you comprehend the meaning 
of a luminous body^ an opaque body^ and a transparent body. 

Caroline, A luminous body is one that shines ; an 
opaque .... 

Mrs, B. Do not proceed to the second, until we have 
agreed upon the definition of the first. All bodies that shine 
are not luminous ; for a luminous body is one that shines by 
its own light, as the sun, the fire, a candle, &c.* 

Emily, Polished metal then, when it shines with so much 
brilliancy, is not a luminous body ? 

Mrs, B. No, for it would be dark if it did not receive 
light from a luminous body ; it belongs, therefore, to the class 

* The direct light of the sun is calculated to be equal to that of 6560 
candles, placed at the distance of one foot from the object ; and that of 
the moon, to the light of one candle at 7 J feet distance ; of Jupiter s.t 
1620 feet, and of Venus at 421 fee^. 



Fi^. 1. 



PZATE :nr 






F^. S. 




ON OPTICS. 167 

of opaque or dark bodies, which comprehend all such as are 
neither luminous nor will admit the light to pass through 
them. 

Emily. And transparent bodies, are those which admit 
the light to pass through them ; such as glass and water ? 

Mrs, B. You are right. Transparent or pellucid bodies, 
are frequently called mediums ; and the rays of light which 
pass through them, are said to be transmitted by them. 

Light, when emanated from the sun, or any other luminous 
body, is projected forwards in straight lines in every possible 
direction ; so that the luminous body is not only the general 
centre from whence all the rays proceed, but every point of 
it may be considered as a centre which radiates light in every 
durection. (fig. 1. plate XV.) 

Emily, But do not the rays which are projected in differ- 
ent directions, and cross each other, interfere, and impede 
each other's course ? 

Mrs, B, Not at all. The particles of light are so ex- 
tremely minute, that they are never known to interfere with 
each other. A ray of light is a single line of light projected 
from a luminous body ; and a pencil of rays, is a collection of 
rays, proceeding from any one point of a luminous body, as 
fig. 2. 

Caroline, Is light then a substance composed of particles 
like other bodies ? 

Mrs, B, This is a disputed point upon which I cannot 
pretend to decide. In some respects, light is obedient to the 
laws which govern bodies ; in others it appears to be inde- 
pendent of them: thus though its corrse is guided by the 
laws of motion, it does not seem to be influenced by those of 
gravity. It has never been discovered to have weight, though 
a variety of interesting experiments have been made with a 
view of ascertaining that point ; but we are so ignorant of 
the intimate nature of light, that an attempt to investigate it 
would lead us into ^ labyrinth of perplexity, if not of error ; 
we shall therefore confine our attention to those properties of 
light which are well ascertained. 

Let us return to the examination of the effects of the radia- 
tion of light from a luminous body. Since the rays of light 
are projected in straight lines, when they meet with an opaque 
body through which they are unable to pass, they are stop- 
ped short in their course 5 for they cannot move in a curve 
line round the body. 



168 ON OPTICS. 

Caroline, No, certainly ; for it would require some other 
force besides that of projection, to produce motion in a curve 
line. 

Mrs. B. The interruption of the rays of light, by the 
opaque body, produces, therefore, darkness on the opposite 
side of it ; and if this darkness fall upon a wall, a sheet of 
paper, or any object whatever, it forms a shadow. 

Emily, A shadow then is nothing more than darkness 
produced by the intervention of an opaque body, which 
prevents the rays of light from reaching an object behind the 
opaque body. 

Caroline, Why then are shadows of different degrees of 
darkness ; for I should have supposed from your definition of 
a shadow, that it would have been perfectly black ? 

Mrs, B, It frequently happens that a shadow is produced 
by an opaque body interrupting the course of the rays from 
one luminous body, while light from another reaches the 
space where the shadow is formed, in wliich case the shadow- 
is proportionally fainter. This happens if the opaque body 
be lighted by two candles : if you extinguish one of them^ 
the shadow will be both deeper and more distinct. 

Caroline. But yet it will not be perfectly dark. 

Mrs. B, Because it is still slightly illumined by light 
reflected from the walls of the room, and other surrounding 
objects. 

You must observe, also, that when a shadow is produced 
by the interruption of rays from a single luminous body, the 
darkness is proportional to the intensity of the light. 

Emily. I should have supposed the contrary ; for as the 
light reflected from surrounding objects on the shadow, must 
be in proportion to the intensity of the light, the stronger the 
light, the more the shadow will be illumined. 

Mrs. B. Your remark is perfectly just ; but as we have 
no means of estimating the degrees of light and of darkness 
but by comparison, the strongest light will appear to produce 
the deepest shadow. Hence a total eclipse of the sun occa- 
sions a more sensible darkness than mid-night, as it is imme- 
diately contrasted with the strong light of noon-day. 

Caroline. The re-appearance of the sun after an eclipse, 
must by the same contrast, be remarkably brilliant. 

Mrs. B. Certainly. There are several things to be ob- 
served in regard to the form and extent of shadows. If the 
luminous body A (Jig. 3.) is larger than the opaque body B, 



ON OPTICS. 169 

the shadow will gradually diminish in size, till it terminate in 
a point. 

Caroline. This is the case with the shadows of the earth 
and the moon, as the sun which illumines them, is larger than 
either of those bodies. And why is it not the case with the 
shadows of terrestrial objects, which are equally illumined by 
the sun ? but their shadows, far from diminishing, are always 
larger than the object, and increase with the distance from it. 

Mrs. B. In estimating the effect of shadows, we must 
consider the apparent not the re«/ dimensions of the lumin- 
ous body ; and in this point of view, the sun is a small object 
compared wdth the generality of the terrestrial bodies which it 
illumines : and when the luminous body is less than the 
opaque body, the shadow will increase with the distance to 
infinity. All objects, therefore, which are apparently larger 
than the sun, cast a magnified shadow. This will be best 
exemplified, by observing the shadow of an object lighted by 
a candle. 

Emily, I have often noticed, that the shadow of my fig- 
ure against the wall, grows larger as it is more distant from 
me, which is owing, no doubt, to the candle that shines on 
me being much smaller than m3^self ? 

Mrs. B. Yes. The shadow of a figure A, (fig. 4.) varies 
in size, according to the distance of the several surfaces B C 
D E, on which it is described. 

Caroline. I have observed, that two candles produce two 
shadows from_ the same object ; whilst it would appear, from 
what you said, that they should rather produce only half a 
iihadow, that is to say, a very faint one. 

Mrs. B. Th^ number of lights (in different directions) 
while it decreases the intensity of the shadow, increases their 
number which always corresponds with that of the lights ; 
for each light makes the opaque body cast a different shadow, 
as illustrated by fig. 5. It represents a ball A, lighted by 
three candles B, C, D, and you observe the light B produces 
tlie shadow 6, the light C the shadow c, and the light D the 
shadow d. 

Emily. I think we now understand the nature of shadows 
very well ; but pray what becomes of the rays of light which 
opaque bodies arrest in their course, and the interruption of 
which is the occasion of shadows ? 

Mrs. B. Your question leads to a very important property 
of light, Reflection, When rays of light encounter an opaque 
15 



170 ON OPTICS. 

body, which they cannot traverse, part of them are absorbed 
by it, and part are reflected, and rebound just as an elastic 
ball which is struck against a wall. 

Emily, And is light in its reflection governed by the same 
laws as solid elastic bodies ? 

Mrs, B, Exactly. If a ray of light fall perpendicularly 
on an opaque body, it is reflected back in the same line, to- 
wards the point whence it proceeded. If it fall obliquely, it 
is reflected obliquely, but in the opposite direction ; the angle 
of incidence being equal to the angle of reflection. You re- 
collect that law in mechanics ? 

Emily, Oh yes, perfectly. 

Mrs, B, If you will shut the shutters, we shall admit a 
ray of the sun's light through a very small aperture, and I 
can show you how it is reflected. I now hold this mirror, so 
that the ray shall fall perpendicularly upon it. 

Caroline, I see the ray which falls upon the mirror, but 
not that which is reflected by it. 

Mrs. B, Because its reflection is directly retrograde. 
The ray of incidence and that of reflection both being in the 
same line, though in opposite directions, are confounded 
together. 

Emily, The ray then which appears to us single, is really 
double, and is composed of the incident ray proceeding to 
the mirror, and of tl^e reflected ray returning from the 
mirror. 

Mrs, B, Exactly so. We shall now separate them by 
holding the mirror M, (fig. 6.) in such a manner, that the 
incident ray A B shall fall obliquely upon it — you see the 
reflected ray B C, is marching ofl' in another direction. If 
we draw a line from the point of incidence B, perpendicular 
to the mirror, it will divide the angle of incidence from the 
angle of reflection, and you will see that they are equal. 

Emily. Exactly ; and now that you hold the mirror so, 
that the ray falls more obliquely on it, it is also reflected 
more obliquely, preserving the equality of the angles of inci- 
dence and reflection. 

Mrs, B, It is by reflected rays only that we see opaque 
objects. Luminous bodies send rays of light immediately to 
our eyes, but the rays which they send to other bodies are 
invisible to us, and are seen only when they are reflected or 
transmitted by those bodies to our eyes. 

Emily, But have we not just seen the ray of light in it<? 



ON OPTICS. 171 

passage from the sun to the mirror, and its reflettion ? yet 
in neither case were those rays in a direction to enter our 
eyes. 

Mrs, B. No. What you saw was the hght reflected to 
your eyes by small particles of dust floating in the air, and 
on which the ray shone in its passage to and from the 
mirror. 

Caroline, Yet I see the sun sliining on that house yonder^ 
as clearly as possible. 

Mrs, B, Indeed you cannot see a single ray which passes 
from the sun to the house ; you see no rays but those which 
enter your eyes ; therefore it is the rays which are reflected 
by the house to you, and not those which proceed from the 
sun to the house, that are visible to you. 

Caroline. Why then does one side of the house appear 
to be in sunshine, and the other in the shade ? for if I cannot 
see the sun shine upon it, the whole of the house should 
appear in the shade. 

Mrs, B, That side of the house which the sun shines 
upon, reflects more vivid and luminous rays than the side 
which is in shadow, for the latter is illumined only by rays 
reflected upon it by other objects, these rays are therefore 
twice reflected before they reach your sight ; and as light 
is more or less absorbed by the bodies it strikes upon, every 
time a ray is reflected its intensity is diminished. 

Caroline, Still I cannot reconcile myself to the idea, that 
we do not see the sun's rays shining on objects, but only those 
which objects reflect to us. 

Mrs, B. I do not, however, despair of convincing you of 
it. Look at that large sheet of water, can you tell why the sun 
appears to shine on one part of it only ? 

Caroline, No, indeed ; for the whole of it is equally 
exposed to the sun. This partial brilliancy of water has 
often excited my wonder ; but it has struck me more par- 
ticularly by moon-light. I have frequently observed a vivid 
streak of moonshine on the sea, while the rest of the water 
remained in deep obscurity, and yet there was no apparent 
obstacle to prevent the moon from shining on every part of 
the water equally. 

Mrs, B, By moon-light the effect is more remarkable, on 
account of the deep obscurity of the other parts of the water ; 
while by the sun's light the eflect is too strong for the eye to 
be able to contemplate it. 



172 «N OPTICS. 

Caroline. But if the sun really shines on every part of 
that sheet of water^ why does not every part of it reflect rays 
to my eyes ? 

Mrs. B, The reflected rays are not attracted out of their 
natural course by your eyes. The direction of a reflected 
ray, you know, depends on that of the incident ray ; the sun's 
rays, therefore, which fall with various degrees of obliquity 
upon the water, are reflected in directions equally various ; 
some of these will meet your eyes, and you will see them, but 
those which fall elsewhere are invisible to you. 

Caroline. The streak of sunshine, then, which we now 
see upon the water, is composed of those rays which by their 
reflection happen to fall upon my eyes ? 

Mrs. B. Precisely. 

Emily. But is that side of the house yonder^ which ap- 
pears to be in shadow, really illumined by the sun, and its 
rays reflected another way ? 

Mrs. B. No ; that is a diflerent case from the sheet of 
water. That side of the house is really in shadow ; it is the 
west side, which the sun cannot shine upon till the afternoon. 

Emily. Those objects, then, which are illumined by 
reflected rays, and those which receive direct rays from the 
sun, but which do not reflect those rays towards us, appear 
equally in shadow ? 

Mrs. B. Certainly ; for we see them both illumined, by- 
reflected rays. That part of the sheet of water, over which 
the trees cast a shadow, by what light do you see it. 

Emily. Since it is not by the sun's direct rays^ it must be 
by those reflected on it from other objects, and which it again 
reflects to us. 

Caroline. But if we see all terrestrial objects by reflected 
light, (as we do the moon,) why do they appear so bright and 
luminous ? I should have supposed that reflected rays would 
have been dull and faint, like those of the moon. 

Mrs. B. The moon reflects the sun's light with as much 
vividness as any terrestrial object. If you look at it on a 
clear night, it will appear as bright as a sheet of water, the 
walls of a house, or any object seen by daylight and on which 
the sun shines. The rays of the moon are doubtless feeble^ 
when compared with those of the sun; but that would not 
be a fair comparison, for the former are incident, the latter 
reflected rays. 

Caroline. True ? and when we see terrestrial objects by 



TLATE. JOi 




ON OPTICS. 173 

moon-light, the light has been twice reflected^ and is conse- 
quently proportionally fainter. 

Mrs. B. In traversing the atmosphere^ the ra3'S, both of 
the sun and moon, lose some of their light. For though the 
pure air is a transparent medium^ which transmits the rays of 
light freely^ we have observed, that near the surface of the 
earth it is loaded with vapors and exhalations^ by which some 
portion of them are absorbed. 

Caroline, I have often noticed that an object on the 
summit of a hill appears more distinct than one at an equal 
distance in a valley, or on a plain ; which is owing, I sup- 
pose, to the air being more free from vapors in an elevated 
situation, and the reflected rays being consequently brighter. 

Mrs. B. That may have some sensible effect ; but when 
an object on the summit of a hill has a back ground of light 
sky, the contrast with the object makes its outline more 
distinct. 

Caroline. I now feel well satisfied that we see opaque 
objects only by reflected rays : but I do not understand how 
these rays show us the objects from which they proceed ? 

Mrs. B. The rays of light enter at the pupil of the eye, 
and proceed to the retina, or optic nerve, which is situated at 
the back part of the eye-ball ; and there they describe the 
figure, color, and (excepting size) form a perfect representa- 
tion of the object from which they proceed. We shall again 
close the shutters, and admit the light through the small 
aperture, and you will see a picture on the wall, opposite the 
aperture, similar to that which is delineated on the retina of 
the eye. 

Caroline. Oh, how v/onderful ! There is an exact picture 
in miniature of the garden, the gardener at work, the trees 
blown about by the wind. The landscape would be perfect, 
if it were not reversed ; the ground being above, and the sky 
beneath. 

Mrs. B. It is not enough to admire, you must under- 
stand this phenomenon, which is called a camera obscura^ 
from the necessity of darkening the room, in order to exhibit 
it. 

This picture is produced by the rays of light reflected from 
the various objects in the garden, and which are admitted 
through the hole in the window shutter. 

The rays from the glittering weathercock at the top of the 
alcove A, (plate XVL fig. 1.) represent it in this spot a ; 

15* 



;/ 4 ON OPTIC.-V. 

for the weathercock being much higher than the aperture in 
the shutter, only a few of the rays, which are reflected by it 
in an obUquely descending direction, can find entrance there. 
The rays of hght,you know, always move in straight lines ; 
those, therefore, which enter the room in a descending direc- 
tion, will continue their course in the same direction, and 
will, consequently, fall upon the lower part of the wall oppo- 
site the aperture, and represent the weathercock reversed in 
that spot, instead of erect in the uppermost part of the land- 
scape. 

E?nily, And the rays of light from the steps (B) of the 
alcove, in entering the aperture, ascend, and will describe 
those steps in the highest instead of the lowest part of the 
landscape. 

Mrs, B. Observe, too, that the rays coming from the 
alcove, which is to our left, describe it on the wall to the 
right ; while those which are reflected by the walnut-tree C 
D, to our right, delineate its figure in the picture to the left 
c ch Thus the rays, coming in diflerent directions, and 
proceeding always in right lines, cross each other at their 
entrance through the aperture : those which come above 
proceed below, those from the right go to the left, those from 
the left towards the right ; thus every object is represented 
in the picture, as occupying a situation the very reverse of 
that which it does in nature. 

Caroline* Excepting the flower-pot E F, which, though 
its position is reversed, has not changed its situation in the 
landscape. 

Mrs* B. The flower-pot is directly in front of the aper- 
ture ; so that its rays fall perpendicularly upon it, and, con- 
sequently, proceed perpendicularly to the wall, w^here they 
delineate the object directly behind the aperture. 

Emily, And is it thus that the picture of objects is painted 
on the retina of the ej'e ? * 

Mrs, B. Precisely. The pupil of the eye, through which 
the rays of light enter, represents the aperture in the window- 
shutter ; and the image delineated on the retina, is exactly 
similar to the picture on the wall. 

* Take off the sclerotica from the back part of the eye of an ox, or 
othei' animal, and place the eye in the hole of the window -shutter of a 
dark room, with its fore part towards the external objects ; a person in 
the room will through the transparent coat, see the inverted ima^e paint-- 
ed upon the retina. 



ON OPTICS. 175 

Caroline. You do not mean to say, that we see only the 
representation of the object which is painted on the retina- 
and not the object itself ? 

Mrs. B. Ify by sight, you understand that sense by which 
the presence of objects is perceived by the mind, through the 
means of the eyes, we certainly see only the image of those 
objects painted on the retina. 

Caroline. This appears to me quite incredible. 

Mrs. B. The nerves are the only part of our frame ca- 
pable of sensation : they appear, therefore, to be the instru- 
ments which the mind employs in its perceptions ; for a 
sensation always conveys an idea to the mind. Now it is 
known, that our nerves can be affected only by contact ; and 
for this reason the organs of sense cannot act at a distance : 
for instance, we are capable of smelling only particles which 
are actually in contact with the nerves of the nose. We 
have already observed, that the odom' of a flower consists m 
effluvia, composed of very minute particles, which penetrate 
the nostrils, and strike upon the olfactory nerves, which 
instantly convey the idea of smell to the mind. 

Emily. And sound, though it is said to be heard at a 
distance, is, in fact, heard only when the vibrations of the 
air, which convey it to our ears, strike upon the auditory- 
nerve. 

Caroline. There is no explanation required, to prove 
that the senses of feeling and of tasting are excited only by 
contact. 

Mrs. B. And I hope to convince you, that the sense of 
sight is so likewise. The nerves, which constitute the sense 
of sight, are not different in their nature from those of the 
other organs ; they are merely instruments which convey 
ideas to the mind, and can be affected only on contact. Now, 
since real objects cannot be brought to touch the optic nerve, 
the image of them is conveyed thither by the rays of light 
proceeding from real objects, which actually strike upon the 
optic nerve, and form that image which the mind perceives. 

Caroline. While I listen to your reasoning, I feel con- 
vinced ; but when I look upon the objects around, and think 
that I do not see them, but merely their image painted in my 
eyes, my belief is again staggered. I cannot reconcile myself 
to the idea, that I do not really see this book which I hold in 
TTiy hand, nor the words which I read in it. 



176 ON OPTICS. 

Mrs. B. Did it ever occur to you as extraordinary^ that 
you never beheld your own face ? 

Caroline, No ; because I so frequently see an exact 
representation of it in the looking-glass. 

Mrs, B, You see a far more exact representation of ob- 
jects on the retina of your eye: it is a much more perfect 
mirror than any made by art. 

Emily, But is it possible, that the extensive landscape^ 
which I now behold from the window, should be represented 
on so small a space as the retina of the eye ? 

Mrs, B, It would be impossible for art to paint so small 
and distinct a miniature ; but nature works with a surer hand, 
and a more delicate pencil. That power, which forms the 
feathers of the butterfly, and the flowerets of the daisy, can 
alone portray so admirable and perfect a miniature as that 
which is represented on the retina of the eye. 

Caroline, But, Mrs. B., if we see only the image of ob- 
jects, why do we not see them reversed, as you showed us 
they were in the camera obscura ? Is not that a strong 
argument against your theory ? 

Mrs, B, Not an unanswerable one, I hope. The image 
on the retina, it is true, is reversed, like that in the camera 
obscura ; as the rays, unless from a very small object, inter- 
sect each other on entering the pupil, in the same manner as 
they do on entering the camera ol3scura. The scene, however^ 
does not excite the idea of being inverted, because we always 
see an object in the direction of the rays which it sends to us. 

Emily, I confess I do not understand that. 

Mrs. B, It is, I think, a diflicult point to explain clearly. 
A ray which comes from the upper part of an object ; des- 
cribes the image on the lower part of the retina ; but expe- 
rience having taught us, that the direction of that ray is from 
above, we consider that part of the object it represents as 
uppermost. The rays proceeding from the lower part of an 
object fall upon the upper part cflhe retina ; but as we know 
their direction to be from below, we see that part of the object 
they describe as the lowest. 

Caroline, When I want to see an object above me, I look 
up ; when an object below me, I look down. Does not this 
prove that I see the objects themselves ? for if I beheld only 
the image, there would be no necessity for looking up or down^ 
according as the object was higher or lower than myself. 



ON OPTICS. 177 

Mrs. B. I beg your pardon. When you look up to an 
elevated object, it is in order that the rays reflected from it 
should fall upon the retina of your eyes ; but the very cir- 
cumstance of directing your eyes upwards convinces you that 
the object is elevated, and teaches you to consider as upper- 
most the image it forms on the retina, though it is, in fact^ 
represented in the lowest part of it. When yo,. look down 
upon an object, you draw your conclusion from a similar 
reasoning ; it is thus that we see all objects in the direction 
of the rays which reach our eyes. 

But I have a further proof in favor of what I have advanced, 
which I hope will remove your remaining doubts ; I shall, 
however, defer it till our next meeting, as the lesson has beea 
sufficiently long to-day. 



CONVERSATION XV. 



OPTICS— CONTINUED. 

ON THE ANGLE OF VISION, AND THE REFLECTION OF 
MIRRORkS 

Angle of Vision ; Reflection of Plain Mirrors ; Reflection 
of Convex Mirrors ; Reflection of Concave Mirrors. 



CAROLINE. 

Well, Mrs. B., I am very impatient to hear what further 
proofs you have to offer in support of your theory. You must 
allow that it was rather provoking to dismiss us as you did at 
our last meeting. 

Mrs. B, You press so hard upon me with your objections, 
that you must give me time to recruit my forces. 

Can you tell me, Caroline, why objects at a distance appear 
smaller than they really are ? 

Caroline, I know no other reason than their distance. 

Mrs. B. I do not think I have more cause to be satisfied 
with your reasons, than you appear to be with mine. 

We must refer again to the camera obscura to account for 
this circumstance and you will find, that the different apparent 
dimensions of objects at different distances, proceed from our 
seeing, not the objects themselves, but merely their image on 
the retina. Fig. 1 . plate XVII. represents a row of trees, as 
viewed in the camera obscura. I have expressed the direction 
of the rays, from the objects to the image, by lines. Now, 
observe, the ray which comes from the top of the nearest tree, 
and that which comes from the foot of the same tree, meet at 
the aperture, forming an angle of about twenty-five degrees ; 
this is called the angle of vision, under which we see the tree. 








""■■■^ 



M 




ON TH5E ANGLE OP VISION. l79 

These Yays cross each other at the aperture, forming equal 
angles on each side of it, and represent the tree inverted in the 
camera obscura. The degrees of the image are considerably 
smaller than those of the object^ but the proportions are 
perfectly preserved. 

Now let us notice the upper and lower ray, from the most 
distant tree ; they form an angle of not more than twelve or 
fifteen degrees, and an image of proportional dimensions. 
Thus, two objects of the same size, as the two trees of the 
avenue, form figures of different sizes in the camera obscura, 
according to their distance ; or in other words, according to 
the angle of vision under which they are seen. Do you 
understand this ? 

Caroline, Perfectly. 

Mrs. B. Then you have only to suppose that the repre- 
sentation in the camera obscura is similar to that on the retina. 

Now since objects in the same magnitudes appear to be of 
different dimensions, when at different distances from us, let 
me ask you, which it is that we see ; the real objects, which 
we know do not vary in size, or the images, which we know 
do vary according to the angle of vision under which we see 
them ? 

Caroline, I must confess, that reason is in favor of the 
latter. But does that chair at the further end of the room 
form an image on my retina much smaller than this which is 
close to me ? they appear exactly of the same size. 

Mrs, B. I assure you they do not. The experience we 
acquire by the sense of touch corrects the errors of our sight 
with regard to objects within our reach. You are so perfectly 
convinced of the real size of objects which you can handle, 
that you do not attend to their apparent difference. 

Does that house appear to you much smaller than when you 
are close to it ? 

Caroline, No, because it is very near us. 

Blrs. B. And yet you can see the whole of it through one 
of the windows of this room. The image of the house, on 
your retina, must, therefore, be smaller than that of the win- 
dow through which you see it. It is your knowledge of the 
real size of the house which prevents your attending to its 
apparent magnitude. If you were accustomed to draw from 
nature, you would be fully aware of this difference. 

Emily, And pray, what is the reason that, when we look 
'10 an avenue, the trees not only appear smaller as they are 



ISO ON THE ANGLE OF VISION. 

more distant, but seem gradually to approach each other till 
they meet in a point ? 

Mrs. B. Not only the trees, but the road which separates 
the two rows, forms a smaller visual angle, in proportion as it 
is more distant from us ; therefore the width of the road 
gradually diminishes as well as the size of the trees, till at 
length the road apparently terminates in a point, at which the 
trees seem to meet. 

But this eftect of the angle of vision will be more fully 
illustrated by a little model of an avenue, which I have made 
for that purpose. It consists of six trees, leading to a hex- 
agonal temple, and viewed by an eye, on the retina of which 
the picture of the objects is delineated. 

I beg that you will not criticise the proportions ; for though 
the eye is represented the size of life, while the trees are not 
jRore than three inches high, the disproportion does not affect 
the principle, which the model is intended to elucidate. 

Emily. The threads which pass from the objects through 
the pupil of the eye to the retina, are, I suppose, to represent 
the rays of light which convey the image of the objects to the 
retina ? 

Mrs. B. Yes. I have been obliged to limit the rays to a 
very small number, in order to avoid confusion ; there are, 
you see, only two from each tree. 

Caroline. But as one is from the summit, and the other 
from the foot of the tree, they exemplify the different angles 
under which we see objects at different distances, better than 
if there were more. 

Mrs. B. There are seven rays proceeding from the temple, 
one from the summit, and two from each of the angles that 
are visible to the eye, as it is situated ; from these you may 
form a ]ust idea of the difference of the angle of vision of 
objects viewed obliquely, or in front ; for though the six sides 
of the temple are of equal dimensions, that which is opposite 
to the eye is seen under a much larger angle, than those which 
are viewed obliquely. It is on this principle that the laws of 
perspective are founded. 

Emily. I am very glad to know that, for I have lately 
begun to learn perspective, which appeared to me a very 
dry study ; but now that I am acquainted with the prin- 
ciples on which it is founded, I shall find it much more 
interesting. 

Caroline. In drawing a view from nature, then, we do not 



(^K THE ANGLE OF VISION. 181 

copy the real objects, but the image they form on the retma 
of our eyes ? 

Mrs, B, Certainly. In sculpture, we copy nature as she 
really exists ; in painting, we represent her as she appears to 
us. It was on this account that I found it difficult to explain 
by a drawing the efi'ects of the angle of vision, and was under 
the necessity of constructing a model for that purpose. 

Emily. I hope you will allow us to keep this model some 
time, in order-lo study it more completely, for a great deal 
may be learned from it ; it illustrates the nature of the angle 
of vision, the apparent diminution of distant objects, and the 
inversion of the image on the retina. But pray, why are the 
threads that represent the rays of light, colored, the same as 
the objects from which they proceed ? 

Mrs, B, That is a question which you must excuse my 
answering at present, but I promise to explain it to you in due 
time. 

I consent very willingly to your keeping the model, on 
condition that you will make an imitation of it, on tlie same 
principle, but representing different objects. 

We must now conclude the observations that remain to be 
made on the angle of vision. 

If an object, with an ordinary degree of illumination, does 
not subtend an angle of more than two seconds of a degree, 
it is invisible. There are consequently two cases in which 
objects may be invisible, either if they are too small, or sa 
distant as to form an angle less than two seconds of a degree. 

In like manner, if the velocity of a body does not exceed 
20 degrees in an hour, its motion is imperceptible. 

Caroline, A very rapid motion may then be imperceptible, 
provided the distance of the moving body is sufficiently great. 

Mrs, B. Undoubtedly ; for tlie greater its distance the 
smaller will be the angle under which its motion will appear 
to the eye. It is for this reason that the motion of the 
celestial bodies is invisible, notwithstanding their immense 
velocity. 

Emily, I am surprised that so great a velocity as 20 
degrees an hour should be invisible. 

Mrs, B, The real velocity depends altogether on the 
-space comprehended in each degree ; and this space depends 
on the distance of the object, and the obliquity of its path. 
Observe, likewise, that we cannot judge of the velocity of a 
body in motion unless we know its distance ; for supposing 
16 



182 ON THE ANGLE OF VISION. 

two men to set off at the same moment from A and B, (fig. 2.) 
to walk each to the end of their respective lines C and D ; if 
they perform their walk in the same space of time, they must 
have proceeded at a very different rate, and yet to an eye 
situated at E, they will appear to have moved with equal 
velocity : because they will both have gone through an equal 
number of degrees, though over a very unequal length of 
ground. Sight is an extremely useful sense no doubt, but it 
cannot always be relied on, it deceives us both in regard to the 
size and the distance of objects ; indeed our senses would be 
very liable to lead us into error, if experience did not set us 
right. 

Emily. Between the two, I think that we contrive to 
acquire a tolerably accurate idea of objects. 

Mrs. B, At least sufficiently so for the general purposes of 
life. To convince you how requisite experience is to correct 
the errors of sight, I shall relate to you the case of a young 
man who was blind from his infancy, and who recovered his 
sight at the age of fourteen, by the operation of couching. 
At first he had no idea either of the size or distance of objects, 
but imagined that every thing he saw touched his eyes ; and 
it was not till after having repeatedly felt them, and walked 
from one object to another that he acquired an idea of their 
respective dimensions, their relative situations, and their 
distances. 

Caroline. The idea that objects touched his eyes, is how- 
ever not so absurd as it at first appears ; for if we consider 
that v/e see only the image of objects, this image actually 
touches our eyes, 

Mrs. B. That is doubtless the reason of the opinion he 
formed, before the sense of touch had corrected his judgment. 

Caroline, But since an image must be formed on the 
retina of each of our eyes, why do we not see objects double ? 

Mrs. B. The action of the rays on the optic nerve of 
each eye is so perfectly similar, that they produce but a single 
sensation, the mind therefore receives the same idea, from the 
retina of both eyes, and conceives the object to be single. 

Caroline. This is difficult to comprehend, and, I should 
think, can be but conjectural. 

Mrs. B. I can easily convince you that you have a dis- 
tinct image of an object formed on the retina of each eye. 
l^ook at the bell-rope, and tell me do you see it to the right or 
the left of the pole of the fire-skreen .^ 



KEFLECTION OF MIRRORS. 183 

Caroline, A little to the right of h. 

Mrs, B, Then shut your right eye, and you will see it to 
the left of the pole. 

Caroline, That is true indeed ! 

Mrs, B, There are evidently two representations of the 
bell-rope in different situations, which must be owing to an 
image of it being formed on both eyes ; if the action of the 
rays therefore on each retina were not so perfectly similar as 
to produce but one sensation, we should see double, and we 
find that to be the case with many persons who are afflicted 
with a disease in one eye, which prevents the rays of light 
from affecting it, in the same manner as the other. 

Emily. Pray, jMrs. B., when we see the image of an object 
in a looking-glass, why is it not inverted as in the camera 
obscura, and on the retina of the eye ? 

Mrs, B, Because the rays do not enter the mirror by a 
small aperture, and cross each other, as they do at the orifice 
of a camera obscura, or the pupil of the eye. 

When you view yourself in a mirror, the rays from your 
eyes fall perpendicularly upon it, and are reflected in the same 
line ; the image is therefore described behind the glass, and is 
situated in the same manner as the object before it. 

Emily, Yes, I see that it is ; but the looking-glass is not 
nearly so tall as I am, how is it therefore that I can see the 
whole of my figure in it ? 

Mrs, B, It is not necessary that the mirror should be 
more than half your height, in order that you may see the 
whole of your person in it, {fig, 3.) The ray of light C D 
from your eye, which falls perpendicularly on the mirror B D, 
will be reflected back in the same line ; but the ray from your 
feet will fall obliquely on the mirror, for it must ascend in 
order to reach it ; it will therefore be reflected in the line 
D A : and since we view objects in the direction of the re- 
flected rays, which reach the eye, and that the image appears 
at the same distance behind the mirror that the object is be- 
fore it, we must continue the line A D to E, and the line C D 
to F,at the termination of which, the image will be represented. 

Emihj. Then I do not understand vv^hy I should not see 
the whole of my person in a much smaller mirror, for a ray 
of light from my feet would always reach it, though more 
obliquely. 

Mrs. B. True ; but the more obliquely the ray falls on 
the mirror, the more obliquely it will be reflected , the ray 



184 REFLECTION OF MIRKOlliJ. 

would therefore be reflected above your head, and you could 
not see it. This is shown by the dotted line, {fig, 3.) 

Now stand a little to the right of the mirror, so that the 
rays of light from your figure may fell obliquely on it — 

Emily. There is no image formed of me in the glass now. 

Mrs, B, I beg your pardon, there is ; but you cannot see 
it, because the incident rays falling obliquely on the mirror 
will be reflected obliquely in the opposite direction, the angles 
of incidence and of reflection being equal. Caroline, place 
yourself in the direction of the reflected rays, and tell me 
whether you do not see Emily's image in the glass ? 

Caroline, Let me consider. In order to look in the di- 
rection of the reflected rays, I must place myself as much to 
the left of the glass as Emily stands to the right of it. Now 
I see her image but it is not straight before me, but before 
her ; and appears at the same distance behind the glass, as 
she is in front of it. 

Mrs, B, You must recollect, that we always see objects 
in the direction of the last rays which reach our eyes. Figure 
4 represents an eye looking at the image of a vase reflected 
by a mirror ; it must see it in the direction of the ray A B, as 
that is the ray which brings the image to the eye : prolong 
the ray to C, and in that spot will the image appear. 

Caroline, I do not understand why a looking-glass reflects 
the rays of light : for glass is a transparent body y/hich should 
transmit them ? -^^ 

Mrs, B. It is not the glass that reflects the rays which 
form the image you behold, but the mercury behiiid it. The 
glass acts chiefly as a transparent case, through which the 
rays find an easy passage. 

Caroline, Why then should not mirrors be made simply 
of mercury ? 

Mrs. B, Because mercury is a fluid. By amalgamating 
it^^kh tin-foil, it becomes of the consistence of paste, attaches 
its^ to the glass, and forms in fact a mercurial mirror, which 
would be much more perfect without its rrlass cover, for the 
purest glass is never perfectly transparent ; some of the rays 
therefore are lost during their passage through it, by being 
either absorbed, or irregularly reflected. 

This imperfection of glass mirrors has introduced the use 
of metallic mirrors, for opiical purposes. 

Emily, But since all opaque bodies reflect the rays of 
light, I do not understand why they are not all mirrors ? 



REFLECTION OF CONVEX MIRRORS. 185 

Caroline, A curious idea indeed, sister ; it would be 
very gratifying to see one's self in every object at which one 
looked. 

Mrs, B, It is very true that all opaque objects reflect 
light ; but the surface of bodies in general is so rough and 
uneven, that their reflection is extremely irregular, which 
prevents the rays from forming an image on the retina. This 
you will be able to understand better, when I shall explain 
to 3^ou the nature of vision, and the structure of the eye. 

You may easily conceive the variety of directions in which 
rays would be reflected by a nutmeg-grater, on account of the 
inequality of its surface, and the number of holes with which 
it is pierced. All solid bodies resemble the nutmeg-grater in 
these respects, more or less ; and it is only those which are 
susceptible of receiving a polish, that can be made to reflect 
the rays with regularity. As hard bodies are of the closest 
texture, the least porous, and capable of taking the highest 
polish, they make the best mirrors ; none therefore are so 
well calculated for this purpose as metals. 

Caroline, But the property of regular reflection is not 
confined to this class of bodies ; for I have often seen myself 
in a highly polished mahogany table. 

Mrs. B, Certainly ; but as that substance is less durable^ 
and its reflection less perfect, than that of metals, I believe it 
would seldom be chosen for the purpose of a mirror. 

There are three kinds of mirrors used in optics ; the plain 
or flat, which are the common mirrors we have just mention- 
ed ; convex mirrors ; and concave mirrors. The reflection 
of the two latter is very difterent from that of the former. 
The plain mirror, we have seen, does not alter the direction 
of the reflected rays, and forms an image behind the glass 
exactly similar to the object before it. A convex mirror has 
the peculiar property of making the reflected rays diverge^ 
by which means it diminishes the image ; and a concave 
mirror makes the rays converge, and, under certain circum- 
stances, magnifies the image. 

Emily, We have a convex mirror in the drawing-room,, 
which forms a beautiful miniature picture of the objects in 
the room ; and 1 have often amused myself with looking at 
my magnified face in a concave mirror. But I hope you will 
explain to us why the one enlarges while the other diminishes 
the objects it reflects. 

Mrs, B, Let us begin by examining the reflection of a 
16^ 



tbO KEI LECTION OF CONVEX MIEKORS. 

convex mirror. This is formed of a portion of the exterior 
surface of a sphere. When several parallel rays fall upon it, 
that ray only, which, if prolonged, would pass through the 
centre or axis of the mirror, is pei*pendicular to it. In order 
to avoid confusion, I have, in fig. 1 . plate XVIII. drawn only 
three parallel lines, A B, C D, E F, to represent rays falling 
on the convex mirror M N ; the middle ray, you will ob- 
serve^ is perpendicular to the mirror, the others fall on it 
obliquely. 

Caroline, As the three rays are parallel, why are they not 
all perpendicular to the mirror ? 

Mrs, B, They would be so to a flat mirror ; but as this is 
sphericau no ray can fall perpendicularly upon it which is not 
directed towards the centre of the sphere. 

Emily, Just as a weight falls perpendicularly to the earth 
when gravity attracts it towards the centre. 

Mrs, B, In order, therefore, that rays may fall perpen- 
dicularly to the mirror at B and F, the rays must be in the 
direction of the dotted lines, which, you may observe, meet 
at the centre O of the sphere, of which the mirror forms a 
portion. 

Now can you tell me in what direction the three rays* 
A B, C D, E F, will be reflected ? 

Emily, Yes, I think so : the middle ray falling perpen- 
dicularly on the mirror, will be reflected in the same line : the 
two others falling obliquely, will be reflected obliquely to 
G H ; for the dotted lines you have drawn are perpendiculars- 
which divide their angles of incidence and reflection. 

Mrs, B, Extremely well, Emily : and since we see ob- 
jects in the direction of the reflected ray, we shall see the 
image at L, which is the point at which the reflected rays, if 
continued through the mirror, would unite and form an image. 
This point is equally distant from the surface and centre of 
the sphere, and is called the imaginary focus of the mirror. 

Caroline, Pray, what is the meaning of focus ? 

Mrs, B, A point at which converging rays unite. And 
it is in this case called an imaginary focus ; because the rays 
do not really unite at that point, but only appear to do so : for 
f he rays do not pass through the mirror, since they are reflected 
by it. 

Emily. I do not yet understand why an object appears 
^mailer when viewed in a convex mirror. 

^frp, /?. It is owing to the divergence of the reflected 



PLATE. XWL 




REFLECTION OF CONCAVE MIRUORS. ISf 

rays. You have seen that a convex mirror converts, by 
reflection, parallel rays into divergent rays ; rays that fail 
upon the mirror divergent, are rendered still more so by 
reflection, and convergent rays are reflected either parallel, 
or less convergent. If then an object be placed before any 
part of a convex mirror, as the vase A B, flg. 2. for instance, 
the two rays from its extremities, falling convergent on the 
mirror, will be reflected less convergent, and will not come to 
a focus till they arrive at C ; then an eye placed in the direc- 
tion of the reflected rays, will see the image formed in (or 
rather behind) the mirror at a h, 

Caroline, But the reflected rays do not appear to me to 
converge less than the incident rays. 1 should have supposed 
that, on the contrary, they converged more, since they meet 
in a point ? 

Mrs. B. They would unite sooner than they actually do^ 
if they were not less convergent than the incident rays : for 
observe, that if the incident rays, instead of being reflected by 
the mirror, continued their course in their original direction^ 
they would come to a focus at D, which is considerably nearer 
to the mirror than at C ; the image is therefore seen under a 
smaller angle than the object ; and the more distant the latter 
is from the mirror, the less is the image reflected by it. 

You will now easily understand the nature of the reflection 
of concave mirrors. These are formed of a portion of the 
internal surface of a hollow sphere, and their peculiar proper- 
ty is to converge the rays of light. 

Can you discover, Caroline, in what direction the three 
parallel rays, A B, C D, E F, which fall on the concave mir- 
ror M N (fig. 3.) are reflected ? 

Caroline, I believe I can. The middle ray is sent back 
in the same line, as it is in the direction of the axis of the 
mirror ; and the two others will be reflected obliquely, as they 
fall obliquely on the mirror. I must now draw two dotted 
lines perpendicular to their points of incidence, which will 
divide their angles of incidence and reflection ; and in order 
that those angles may be equal, the two oblique rays must be 
reflected to L, where they will unite with the middle ray. 

Mi's. B. Very well explained. Thus you see that when 
any number of parallel rays fall on a concave mirror they are 
all reflected to a focus ; for in proportion as the rays are 
more distant from the axis of the mirror, they fall more ob- 
liquely upon it, and are more obliquely reflected ; in conse- 



188 REFLECTION OF CONCAVE MIRRORS. 

quence of which they come to a focus in the direction of the 
axis of the mirror, at a point equally distant from the centre 
and the surface of the sphere, and this point is not an imagina- 
ry focus, as happens with the convex mirror, but is the true 
focus at which the rays unite. 

Emihj, Can a mirror form more than one focus by reflect- 
ing rays? 

Mrs, B, Yes. If rays fall convergent on a concave mir- 
ror, (fig. 4.) they are sooner brought to a focus, L, than par- 
allel rays ; their focus is therefore nearer to the mirror M 
N. Divergent rays are brought to a more distant focus than 
parallel rays, as in fig. 5. where the focus is at L ; but the 
true focus of mirrors, either convex or concave, is that of 
parallel rays, which is equally distant from the centre, and 
the surface of the sphere. 

I shall nov/ show you the reflection of real rays of light, 
by a metallic concave mirror. This is one made of polished 
tin, which I expose to the sun, and as it shines bright, we 
shall be able to collect the rays into a very brilliant focus. 
I hold a piece of paper where I imagine the focus to be situ- 
ated ; 3 ou may see by the vivid spot of light on the paper, 
how much the rays converge : but it is not yet exactly in the 
focus ; as I approach the paper to that point, observe hov/ 
the brightness of the spot of light increases^ while its size 
diminishes. 

Caroline, That must be occasioned by the rays becoming 
closer together. I think you hold the paper just in the focus 
now, the light is so small and dazzling — Oh, Mrs. B., the 
paper has taken fire ! 

Mrs, B, The rays of light cannot be concentrated, with- 
out, at the same time, accumulating a proportional quantity 
of heat : hence concave mirrors have obtained the name of 
burning-mirrors. 

Ennly, I have often heard of the surprising effects of 
burning-mirrors, and I am quite delighted to understand their 
nature. 

Caroline, It cannot be the true focus of the mirror at 
which the rays of the sun unite, for as they proceed from a 
point, they must fall divergent upon the mirror. 

Mrs, B, Strictly speaking, they certainly do. But when 
rays come from such an immense distance as the sun, their 
divergence is so trifling, as to be imperceptible ; and they 
may be considered as parallel : their point of union is, there- 



THE REFLECTION OF MIRRORS. 189 

fore, the true focus of the mirror^ and there the image of the 
object is represented. 

Now that I have removed the mirror out of the influence of 
the sun's rays, if I place a burning taper in the focus, how 
will its light be reflected ? (iig. 6.) 

Caroline* That, I confess, I cannot say. 

Mrs. B, The ray which falls in the direction of the axis 
of the mirror, is reflected back in the same line ; but let us 
drav>^ tv>^o other rays from the focus, falling on the mirror at 
B and F ; the dotted linob are perpendicular to those points, 
and die two rays will thereibre be reflected to A and E. 

Caroline, Oh, now I understand it clearly. The rays 
which proceed from a light placed in the focus of a concave 
mirror fall divergent upon it, and are reflected parallel. It is 
exactly the reverse of the former experiment, in which the sun's 
rays fell parallel on the mirror, and were reflected to a focus. 

Mrs. B, Yes : when the incident rays are parallel, the 
reflected rays, converge to a focus ; when, on the contrary, 
the incident rays proceed from the focus, they are reflected 
parallel. This is an important law of optics, and since you 
are now acquainted with the principles on which it is founded, 
I hope that you will not forget it. 

Caroline, I am sure that we shall not. But, Mrs. B., 
you said that the image was formed in the focus of a concave 
mirror ; yet I have frequently seen glass concave mirrors, 
where the object has been represented within the mirror, in 
the same manner as in a convex mirror. 

Mrs, B, That is the case only, when the object is placed 
between the mirror and its focus ; the image then appears 
magnified behind, or, as you call it, within the mirror. 

Caroline, I do not understand why the image should be 
larger than the object, 

Mrs. B, It proceeds from the convergent property of the 
concave mirror. If an object, A B, (fig. 7.) be placed between 
the mirror and its focus, the rays from its extremities fall 
divergent on the mirror, and on being reflected, becomes less 
divergent, as if they proceeded from C : to an eye placed in 
that situation the image will appear magnified behind the mirror 
at a 6, since it is seen under a larger angle than the object. 

You now, I hope, understand the reflection of light by 
opaque bodies. At our next meeting, we shall enter upon 
another property of light no less interesting which is called 
refraction. 



CONVERSATION XVI. 



ONREFRACTIOxN AND COLORS. 

Transmission of Light by Transparent Bodies ; Refrac- 
tion ; Refraction of the Atmosphere ; Refraction of a 
Lens ; Refraction of the Frism ; Of the Colors of Rays 
of Light ; Of the Colors of Bodies, 



MRS. B. 

The refraction of light will furnish the subject of to-day's 
tesson. 

Caroline, That is a property of which I have not the 
iaintest idea. 

Mrs. jB. It is the effect which transparent mediums pro- 
duce on light in its passage through them. Opaque bodies, 
you know, reflect the rays, and transparent bodies transmit 
them ; but it is found, that if a ray, in passing from one 
medium into another of different density, fall obliquely, it is 
turned out of its course. 

Caroline* It must then be acted on by some new power, 
otherwise it would not deviate from its first direction. 

Mrs. B, The power which causes the deviation of the 
ray appears to be the attraction of the denser medium. Let 
us suppose the two mediums to be air and water ; if a ray of 
light passes from air into water it is more strongly attracted 
by the latter on account of its superior density. 

Emily. In what direction does the water attract the ray ? 

Mrs. B. It must attract it perpendicularly towards it, in 
the same manner as gravity acts on bodies. 

If then a ray A B, (fig. 1. plate XIX.) fall perpendicularly 
on water, the attraction of the water acts in the same direction 
as the course of the ray : it will not therefore cause a devia« 



Fiff. 1. 



A''- ^ TLATE.Xn: 




E ]f D 




Fiq. 4. 




T«E REFRACTION OF LIGHT. 191 

tioiij and the ray will proceed straight on to E. But if it fall 
obliquely, as the ray C B, the water will attract it out of its 
course. Let us suppose the ray to have approached the sur- 
face of a denser medium, and that it there begins to be af- 
fected by its attraction ; this attraction, if not counteracted 
by some other power, would draw it perpendicularly to the 
water, at B ; but it is also impelled by its projectile force, 
which the attraction of the denser medium cannot overcome ; 
the ray, therefore, acted on by both these powers, moves in a 
direction between them, and instead of pursuing its original 
course to D, or being implicitly guided by the water to E, 
proceeds towards F, so that the ray appears bent or broken. 

Caroline, I understand that very well ; and is not this 
the reason that oars appear bent in water ? 

Mrs. B, It is owing to the refraction of the rays reflected 
by the oar ; but this is in passing from a dense to a rare me- 
dium, for you know that the rays, by means of which you see 
the oar, pass from water into air, 

Emily. But I do not understand why a refraction takes 
place when a ray passes from a dense into a rare medium ; I 
should suppose that it would be rather less, than more 
attracted by the latter. 

Mrs. B. And it is precisely on that account that the ray 
is refracted. C B, fig. 2. represents a ray passing obliquely 
from glass into water : glass being the denser medium, the 
ray will be more strongly attracted by that which it leaves 
than by that which it enters. The attraction of the glass 
acts in the direction A B, while the impulse of projection 
would carry the ray to F ; it moves therefore between these 
directions towards D. 

Emily. So that a contrary refraction takes place when a 
ray passes from a dense into a rare medium. 

Caroline. But does not the attraction of the denser medium 
affect the ray before it touches it ? 

Mrs. B. The distance at which the attraction of the den- 
ser medium acts upon a ray is so small as to be insensible ; it 
appears therefore to be refracted only at the point at which it 
passes from one medium to the other. 

Now that you understand the principle of refraction, I will 
show you Jhe refraction of a real ray of light. Do you see 
the flower painted at the bottom of the inside of this tea-cup ? 
(Fig. 3.) 



102 THE REFRACTION OP LIGHT. 

Emily. Yes. But now you have moved it just out of 
sight, the rim of the cup hides it. 

Mrs. B. Do not stir. I will fill the cup with water, and 
you will see the flower again. 

Emily. I do indeed ! Let me try to explain this : when 
you drew the cup from me so as to conceal the flower, the 
rays reflected by it no longer met my eyes, but were directed 
above them ; but now that you have filled the cup with water, 
they are refracted by the attraction of the water, and bent 
downwards so as again to enter my eyes. 

Mrs. B. You have explained it perfectly : Fig. 3. wiir 
help to imprint it on your memory. You must observe that 
when the flower becomes visible by the refraction of the ray, 
3^ou do not see it in the situation which it really occupies, but 
an image of the flower higher in the cup ; for as objects always 
appear to be situated in the direction of the rays which enter 
the eye, the flower will be seen in the direction of the reflected 
ray at B. 

Emily, Then, when we see the bottom of a clear stream 
of water, the rays which it reflects being refracted in their 
passage from the water into the air, will make the bottom 
appear higher than it really is. 

Mrs. B. And the water will consequently appear more 
shallow. Accidents have frequently been occasioned by this 
circumstance ; and boys who are in the habit of bathing 
should be cautioned not to trust to the apparent shallowness 
of water, as it will always prove deeper than it appears ; 
unless, indeed, they view it from a boat on the water, which 
will enable them to look perpendicularly upon it ; when the 
rays from the bottom passing perpendicularly, no refraction 
will take place. 

The refraction of light prevents our seeing the heavenly 
bodies in their real situation ; the light they send to us being 
refracted in passing into the atmosphere, we see the sun and 
stars in the direction of the refracted ray ; as described in fig. 
4. plate XIX., the dotted line represents the extent of the 
atmosphere, above a portion of the earth, E B E : a ray of 
light coming from the sun S falls obliquely on it, at A, and is 
refracted to B : then, since we see the object in the direction 
of the refracted ray, a spectator at B will see an image of the 
sun at C, instead of the real object at S. 

Emily. But if the sun were immediately over our heads, 



THE REFRACTION OP LIGHT. . 193 

it s rays failing perpendiculariy on tlie atmosphere would not 
be refractedj and we sliould tlien see the real sun, in its true 
situation. 

Mi's, B. You must recollect that the sun is vertical only 
to the inhabitants of the torrid zone ; its rays, thereforCj are 
always refracted in these climates. There is also anotliier 
obstacle to our seeing the heavenly bodies in their real situa- 
tions ; light, though it moves with extreme velocity, is about 
eight minutes and an half in its passage from the sun to the 
earth : therefore, when the rays reach us, the siui must have 
quitted the spot he occupied on their departure ; yet we see 
him in the direction of those rays, and consequently in a 
situation wliich he had abandoned eight minutes and an half 
before. 

Emily, When you speak of the sun's motion, you mean, 
I suppose, his apparent motion, produced by the diurnal 
motion of the earth ? 

Mrs, B. No doubt ; the eflect being the same, whether 
it is our earth, or the heavenly bodies which move : it is 
more easy to represent things as they appear to be, than as 
the}^ realh" are. 

Caroline. During the morning, then, when the sun is 
rising towards the meridian, v/e must (from the length of time 
the light is in reaching us) see an image of the sun below that 
spot which it really occupies. 

Emily. But the refraction of the atmosphere counteract- 
ing this effect, we may perhaps, between the two, see the sun 
in its real situation. 

Caroline. And in the afternoon, when the sun is sinking 
in the west, refraction and the length of time which the light 
is in reaching the earth, will conspire to render the image of 
the sun higher than it really is. 

Mrs. B. The refraction of the sun's rays by the atmos- 
phere prolongs our days, as it occasions our seeing an image 
of the sun both before he rises and after he sets ; for below 
the horizon, he still shines upon the atmosphere, and his rays 
are thence refracted to the earth. So likewise we see an 
image of the sun before he rises, the rays that previously fall 
upon the atmosphere being reflected to the earth. 

Caroline. On the other hand we must recollect that light 
is eight minutes and an half on its journey ; so that, by the 
time it reaches the earth, the sun may perhaps be risen above 
the horizon. 

17 



194 THE REFRACTION OF LIGHT. 

Emily, Pray, do not glass windows refract the llglit r 

Mrs, B. They do ; but this refraction is not perceptible, 
because, in passing through a pane of glass the rays suffer 
two refractions, wiiich being in contrary directions, produce 
the same effect as if no refraction had taken place. 

Emily, I do not understand that. 

Mrs, B, Fig. 5. plate XIX. will make it clear to you : 
A A represents a thick pane of glass seen edgeways. When 
the ray B approaches tlie glass at C, it is refracted by it ; 
and instead of continuing its course in the same direction, as 
the dotted line describes, it passes through the pane to D ; at 
that point returning into the air, it is again refracted by the 
glass, but in a contrary direction to the first refraction, and 
in consequence proceeds to E. Now you must observe that 
the ray B C and the ray D E being parallel, the light does not 
appear to have suffered any refraction. 

Emily, So that the effect v/hich takes place on the ray 
entering the glass, is undone on its quitting it. Or, to express 
myself more scientifically, when a ray of light passes from 
one medium into another, and through that into the first again, 
the two refractions being equal and in opposite directions, no 
sensible effect is produced. 

Mr«. B, This is the case when the two surfaces of the 
refracting medium are parallel to each other ; if they are not, 
the two refractions may be made in the same direction, as I 
shall show you. 

When parallel rays (fig. 6.) fall on a piece of glass having 
a double convex surface, and which is called a Lens^ that 
only which falls in the direction of the axis of the lens is 
perpendicular to the surface ; the other rays falling obUquely 
are refracted towards the axis, and will meet at a point be- 
yond the lens called its focus. 

Of the three rays, ABC, which fall on the lens D E, the 
rays A and C are refracted in their passage through it, to a, 
and c, and on quitting the lens they undergo a second refrac- 
tion in the same direction which unites them with the ray B, 
at the focufej F. 

Emily, And what is the distance of the focus from the 
surface of the lens ? 

Mrs, B, The focal distance depends both upon the form 
of the lens, and of the refractive power of the substance of 
which it is made : in a glass lens, both sides of which are 
equally convex, the focus is situated nearly at the centre of 



PLATE XL 





ON KEFRAC TION AND COLORS. 195 

the Sphere of which the surface of the lens forms a portion ; 
it is at the distance^ therefore, of the radius of the sphere. 

There are lenses of various forms, as you will find describ- 
ed in fig. 1. plate XX. The property of those which have a 
convex surface is to collect the rays of light to a focus ; and 
of those which have a concave surface, on the contrary, to 
disperse them. For the rays A C falling on the concave 
lens X Y, (fig. 7. plate XIX.) instead of converging towards 
the ray B, which fails on the axis of the lens, will each be 
attracted tow^ards the thick edges of the lens, both on entering 
and quitting it, and will, therefore, by the first refraction, be 
made to diverge to a, r, and by the second to d, e. 

Caroline, And lenses which have one side flat and the 
otiier convex or concave, as A and B, fig. 1, plate XX., are, 
I suppose, less powerful in their refractions ? 

Mrs, B. Yes ; they are called plano-convex, and plano- 
concave lenses : the focus of the former is at the distance of 
the diameter of a sphere, of which the convex surface of the 
lens forms a portion ; as represented in fig. 2. plate XX. 
The three parallel rays, ABC, are brought to a focus by the 
plano-convex lens, X Y at F. 

I must now explain to you the refraction of a triangular 
pieceof glass, called a prism. (Fig. 3.) 

Emily, The three sides of this glass are flat ; it cannot 
therefore bring the rays to a focus ; nor do I suppose that its 
refraction will be similar to that of a flat pane of glass, 
because it has not two sides parallel ; I cannot there- 
fore conjecture what eflect the rehaction of a prism can 
produce. 

Mrs. B, The refractions of the light, on entering and on 
quitting the prism, are both in the same direction. (Fig. 3.) 
On entering the prism F, the ray A is refracted from B to C, 
and on quitting it from C to D. 

I will show you this in nature ; but for this purpose it will 
be advisable to close the window-shutters, and admit, through 
the small aperture, a ray of light, which I shall refract by 
means of this prism. 

Caroline, Oh, what beautiful colors are represented on 
the opposite wall ! There are all the colors of the rainbow, 
and with a brightness I never saw equalled. (Fig. 4. plate 
Xa. j 

Emihj, I have seen an effect, in some respect similar to 
this, produced by tjie rays of the sun shining upon glass 



19G ON REFilACTION AND COLOKS, 

lustres ; but how is it possible that a piece of wiiite glass cun 
produce such a variety of brilliant colors ? 

Mrs. ii. The colors are not formed by the prism, but 
existed in the ray previous to its refraction. 

Caroline, Yet^ before its refraction, it appeared perfectly 
wliite. 

Mrs, B. The white rays of the sun are composed of 
colored rays, which, v/hen blended together, appear colorless 
or white. 

Sir Isaac Newton., to whom we are indebted for the most 
important discoveries respecting light and colors, was the 
first who divided a white ray of light, and found it to consist 
of an assemblage of colored rays, which formed an image 
upon the wall, such as you now see exhibited, (fig. 4.) in 
which are displa3'ed the following series of colors : red, 
orange, yellow, green, blue, indigo, and violet. 

Erailij, But how does a prism separate these colored rays ? 

Mrs, B. By refraction. It appears that the colored rays 
have different degrees of refrangibility ; in passing through 
the prism, therefore, they take different directions according 
to their susceptibility of refraction. The violet rays deviate 
most from their original course ; they appear at one of the 
ends of the spectrum A B : contiguous to the violet, are the 
blue rays, being those which have somewhat less refrangi- 
bility ; then follow, in succession, the green, yellow, orange, 
and, lastly, the red, which are the least refrangible of the 
colored rays. 

Caroline, I cannot conceive how these colors, mixed 
together, can become white ? 

Mrs. B, That I cannot pretend to explain ; but it is a 
fact that the union of these colors, in the proportions in which 
they appear in the spectrum, produce in us the idea of white- 
ness. If you paint a card in compartments with these sevei\ 
colors, and whirl it rapidly on a pin, it will appear white. 

But a more decisive proof of the composition of a white ray 
is afforded by reuniting these colored rays, and forming with 
them a ray of white light. 

Caroline, If you can take a ray of white light to pieces, 
and put it together again, I shall be quite satisfied. 

Mis, B, This can be done by letting the colored rays, 
which have been separated by a prism, fall upon a lens, 
which will converge them to a focus ; and if, when thus 
reunited, we find that they appear white as they did before 



ON REFHACTION AND COLOrvS. 197 

refractioDj I hope that you will be convinced that the white 
rays are a compound of the several colored rays. The prism 
P, 3^ou see, (iig. 5.) separates a ray of white light into seven 
colored rays, and the lens L L brings them to a focus at F, 
where they again appear v/hite. 

Caroline. You succeed to perfection : this is indeed a 
most interesting and conclusive experiment. 

Emily. Yet, Mrs. B., I cannot help thinking, that there 
may perhaps be bat three distinct colors in the spectrum, red, 
yellovv^, and blue ; and that the four others may consist of 
two of these colors blended together ; for, in painting, we 
find that by mixing red and yellovv^, we produce orange ; 
with different proportions of red and blue, we make violet or 
any shade of purple ; and yellow and blue form green. 
Now it is very natural to suppose, that the refraction of a 
prism may not be so perfect as to separate the colored rays of 
light completely, and that those which are contiguous in or- 
der of refrangibility may encroach on each other, and by mix- 
ing produce the intermediate colors, orange, green, violet, 
and indigo. 

Mrs. B. Your observation is, I believe, neither quite 
wrong, nor quite right. Dr. Wollaston, who has refracted 
light in a more accurate manner than had been previously 
done, by receiving a very narrow line of light on a prism, 
foiuid that it formed a spectrum, consisting of rays of four 
colors only ; but they were not exactly those you have nam- 
ed as primitive colors, for the}* consisted of red, green, 
blue, and violet. A very narrow line of yellow was visi- 
ble, at the limit of the red and green, which Dr. Wollaston 
attributed to the overlapping of the edges of the red and 
green light. 

Caroline. But red and green mixed together, do not pro- 
duce yellow ? 

Mrs. B. Not in painting ; but it may be so in the primi- 
tive rays of the spectrum. Dr. Wollaston observed that, by 
increasing the breadth of the aperture by which the line of 
light was admitted, the space occupied by each colored ray 
in the spectrum was augmented, in proportion as each por- 
tion encroached on the neighboring color and mixed with it ; 
so that the intervention of orange and yellow, between the 
red and green, is owing, he supposes, to the mixture of these 
two colors, and the blue is blended on the one side with the 
green, and on the other with the violet, forming the spectrum 
17* 



19S uN REFRACTION AND C0L0R3> 

as it was originally observed by Sir Isaac Ncwloiij and which 
I have just shown you. 

The rainbow, which exhibits a series of colors so analo- 
gous to those of the spectrum, is formed by the refraction of 
the sun's rays in their passage through a shower of rain, 
every drop of which acts as a prism, in separating the colored 
rays as they pass through it. 

Emily, Pray, Mrs. B., cannot the sun's rays be collected 
to a focus by a lens in the same manner as they are by a 
concave mirror ? 

Mrs. B, No doubt the same effect is produced by the re- 
fraction of a lens as by the reflection of a concave mirror : 
in the first, the rays pass through the glass and converge to a 
focus behind it ; in the latter, they are reflected from the 
mirror, and brought to a focus before it. A lens, when used 
for the purpose of collecting the sun's rays, is called a burning 
glass. The sun now shines veiy bright ; if we let the rays 
fa!! on this lens you will perceive the focus. 

Emily, Oh yes : the point of union of the rays is very 
luminous. I will hold a piece of paper in the focus, and see 
if it will take fire. The spot of light is extremely brilliant, 
but the paper does not burn ? 

Mrs. B. Try a piece of brown paper ; — that you see 
takes fire almost irnmediatel3\ 

Caroline. This is surprising : for the light appeared to 
shine more intensely on the white than on the brown paper. 

Mrs. B. The lens collects an equal number of rays to a 
focus, whether you hold the v/hite or the brown paper there ; 
but the white paper appears more luminous in the focus, be- 
cause most of the rays, instead of entering into the paper, are 
leflected by it ; and this is the reason that the paper is not 
burnt ; whilst, on the contrary, the brown paper, which ab- 
sorbs more light than it reflects, soon becomes heated and 
takes fire. 

Caroline. This is extremely curious ; but why should 
brown paper absorb more rays than white paper ? 

Mrs. B. I am far from being able to give a satisfactory 
answer to that question. We can form but mere conjecture 
on this point ; and suppose that the tendency to absorb, or 
reflect rays, depends on the arrangement of the minute parti- 
cles of the body, and that this diversity of arrangement ren- 
ders some bodies susceptible of reflecting one colored ray, 
and absorbing the others ; whilst other bodies have a ten- 



ON REFRACTION AND COLORS- 199 

deiicy to reflect all the colors, and others again j to absorb 
them all. 

Emily, And how do you know which colors bodies have 
a tendency to reflect ; cr which to absorb ? 

Mrs, B. Because a body always appears to be of the 
color which it reflects ; for, as we see only by reflected rays^ 
it can appear but of the color of those rays. 

Caroline, But we see all bodies of their own natural 
color, Mrs. B. ; the grass and trees, green ; the sky, blue ; 
the flowers, of various hues. 

Mrs, B, True : but why is the grass green ? because it 
absorbs all except the green rays ; it is therefore these only 
which the grass and trees reflect to our eyes, and which 
makes them appear green. The sky and flovv^ers in the same 
manner, reflect the various colors of which they appear to us ; 
the rose, the red rays ; the violet, the blue ; the jonquil, the 
yellow, Szc, 

Caroline, But these are the permanent colors of the gi'ass 
and flowers, whether the sun's rays shine on them or not. 

Mrs, B. Whenever you see those colors, the flowers 
must be illumined by some light ; and light, from whatever 
source it proceeds, is of the same nature, composed of the 
various colored rays, which paint the grass, the flowers, and 
every colored object in nature. 

Caroline, But, Mrs. B., the grass is green, and the flow- 
ers are colored, whether in the dark, or exposed to the light ? 

Mrs, B, Why should you think so ? 

Caroline, It cannot be otherwise. 

Mrs, B, A most philosophical reason indeed ! But, as I 
never saw them in the dark, you will allow me to dissent 
from your opinion. 

Caroline, What color do you suppose them to be, then, 
in the dark ? 

Mrs, B, None at all ; or black, which is the same thing. 
You can never see objects without light. Light is composed 
of colors, therefore there can be no light without colors ; and 
though every object is black, or without color in the dark, it 
becomes colored, as soon as it becomes visible. It is visible, 
indeed, but by the colored rays which it reflects ; therefore 
we can see it only when colored. 

Caroline, All you say seems very true and I know not 
what to object to it ; yet it appears at the same time i»credi- 



200 ON REFRACTION AND COLORS. 

ble ! What, Mrs. B., are we all as black as negroes, in the 
dark ? you make me shudder at the thought. 

Mrs, B. Your vanity need not be alarmed at the idea, as 
you are certain of never being seen in that slate. 

Caroline. That is some consolation, undoubtedly ! but 
what a melancholy reflection it is, that all nature which ap- 
pears so beautifully diversified with colors should be one 
uniform mass of blackness ! 

Mrs. B. Is nature less pleasing for being colored, as well 
as iliumlned by the rays of light ; and are colors less beautiful, 
for being accidental, rather than essential properties of bodies ? 

Providence appears to have decorated nature with the 
enchanting divei^ity of colors which we so much admire, for 
the sole purpose of beautifying the scene, and rendering it a 
source of pleasurable enjoyment : it is an ornament which 
etnbeliishes nature vvhenever we behold her. What reason is 
there to regret that she does not w^ar it when she is invisible ? 

Emihj. I confess, Mrs. B., that I have had my doubts, as 
well as Caroline, though she has spared me the pains of ex- 
pressing them ; bat I have just thought of an experiment, 
which, if it succeeds, will, I am sure, satisfy us both. It is 
certain, that we cannot see bodies in the dark, to know 
whether they have then any color. But we may place a 
colored body in a ray of light, which has been refracted by a 
prism ; and if your theory is true, the body, of whatever 
color it naturally is, must appear of the color of the ray in 
v/hich it is placed ; for since it receives no other colored rays, 
it can reflect no others. 

Carolbip,. Oh ! that is an excellent thought, Emily ; will 
you stand the test, Mrs. B. 

Mrs. B. I consent : but we must darken the room, and 
admit only the ray which is to be refracted ; otherwise, the 
white rays wall be reflected on the body under trial from various 
parts of the room. With what do you choose to make the 
experiment ? 

Caroline. This rose : look at it, Mrs. B., and tell me 
whether it is possible to deprive it of its beautiful color ? 

Mrs. B. We shall see. — I expose it first to the red rays, 
and the flower appears of a more brilliant hue ; but observe 
the green leaves — 

Caroline. They appear neither red nor green ; but of a 
dingy brown with a reddish glow i 



ON REl RACTiON AND COLORS. 201 

Mrs. B* They cannot be green, because they have no 
green rays to reflect ; neither are they red, because green 
bodies absorb most of the red rays. But though bodies, from 
the arrangement of their particles, have a tendency to absorb 
some rays, and reflect others, yet it is not natural to suppose^ 
that bodies are so perfectly uniform in their arrangement, as 
to reflect only pure rays of one color, and perfectly absorb the 
others ; it is found, on the contrary, that a body reflects, in 
great abundance, the rays w hich determine its color, and the 
others in a greater or less degree, in proportion as they are 
nearer or further from its own color, in the order of refrangi- 
bility. The green leaves of the rose, therefore, will reflect a 
few of the red rays, which, blended with their natural black- 
ness, give them that brown tinge ; if they reflected none of 
the red rays, they would appear perfectly black. Now I 
shall hold the rose in the blue rays — 

Caroline, Oh, Emily, Mrs. B. is right ! look at the rose : 
it is no longer red, but of a dingy blue color. 

Emily, This is the most wonderful of any thing we have 
yet learnt. But, Mrs. B., what is the reason that the green 
leaves are of a brighter blue than the rose ? 

Mrs, B, The green leaves reflect both blue and yellow 
rays, which produces a green color. They are now in a col- 
ored ray, which they have a tendency to reflect ; they^ 
therefore, reflect more of the blue ?ays than tlie rose, (which 
naturally absorbs that color,) and will, of course, appear of a 
brighter blue. 

Emihj, Yet, in passing the rose through the different col« 
ors of the spectrum, the flower takes them more readily than 
the leaves. 

Mrs, B, Because the flower is of a paler hue. Bodies 
which reflect all the rays are white ; those which absorb them 
all are black : between these extremes, the body appears 
lighter or darker, in proportion to the quantity of rays they 
reflect or absorb. This rose is of a pale red : it approaches 
nearer to white than black ; it therefore reflects rays more 
abundantly than it absorbs them. 

Emily, But if a rose has so strong a tendency to reflect 
rays, I should have imagined that it would be of a deep red 
color. 

Mrs, B. I mean to say, that it has a general tendency to 
reflect rays. Pale-colored bodies reflect all the colored rays 
to ^ certain degree, which produces their paleness, approach- 



J02 ON REFRACTION AND COLORS. 

ing to whiteness ; but one color they reflect more than the 
rest ; this predominates over the white, and determines the 
color of the body. Since, then, bodies of a pale color in 
some degree reflect all the rays of light, in passing through 
the various colors of the spectrum, they will reflect them all 
with tolerable brilliancy ; but will appear most vivid in the 
ray of their natural color. The green leaves, on the contra- 
ry, are of a dark color, bearing a stronger resemblance to 
black, than to white ; they have, therefore, a greater tenden- 
cy to absorb, than to reflect rays ; and reflecting very few 
of any but the blue and*yellow rays, they will appear dingy 
in passing through the other colors of the spectrum. 

Caroline, They must, however, reflect great quantities of 
the green rays, to produce so deep a color. 

^ rs. B, Deepness or darkness of color proceeds rather 
from a deficiency than an abundance of reflected rays. Re- 
member that bodies are, of themselves, black ; and if a body 
reflects only a few green rays, it will appear of a dark green ; 
it is the brightness and intensity of the color w hich show that 
a great quantity of rays are reflected. 

Emily, A white body, then, which reflects all the rays^ 
will appear equally bright in all the colors of the spectrum. 

Mrs. B, Certainly ; and this is easily proved by passing 
a sheet of white paper through the rays of the spectrum. 

Caroline, What is the reason that blue often appears 
green by candle-lis^ht ? 

Mrs. B. The light of a candle is not so pure as that of 
the sun ; it has a yellowish tinge, and when refracted by the 
prism, the yellow rays predominate ; and as blue bodies 
reflect the yellow rays in the next proportion, (being next in 
order of refransfibility,) the superabundance of yellow rays 
gives to blue bodies a greenish hue. 

Caroline, Candle-light must then give to all bodies a 
yellowish tinge, from the excess of yellow rays ; and yet it is 
a common remark, that people of a sallow complexion appear 
fairer or whiter by candle-light. 

Mrs. B, The yellow cast of their complexion is not so 
striking, when every object has a yellow tinge. 

Emily, Pray, w^hy does the sun appear red through a fog? 

Mrs. B, It is supposed to be owing to the red rays having 
a greater momentum, which gives them power to traverse so 
dense an atmosphere. For the same reason, the sun generally 
appears red at rising and sotting : as the increased quantity 



ON REFRACTION AND COLORS. 203 

of atmosphere^ which the oblique rays must traverse, loaded 
with the mists and vapors which are usuall}^ formed at those 
times prevents the other rays from reaching us. 

Caroline. And, pray, why are the skies of a blue color ? 

Mrs, B, You should rather say, the atmosphere ; for the 
sky is a very vague term, the meaning of which it would be 
difficult to define philosophically. 

Caroline, But the color of the atmosphere should be 
white, since all the ra3^s traverse it in their passage to the 
earth. 

Mrs. B. Do not forget that we see none of the rays 
which pass from the sun to the earth, excepting those which 
meet our eyes ; and this happens only if we look at the sun, 
and thus intercept the ra3^s, in which case, you know, the sun 
appears white. The atmosphere is a transparent medium, 
through which the sun^s rays pass freely to the earth ; but 
when reflected back into the atmosphere, their momentum is 
considerably diminished ; and they have not all of them 
power to traverse it a second time. The momentum of the 
blue rays is least ; these, therefore, are the most impeded in 
their return, and are chiefly reflected by the atmosphere : this 
reflection is performed in every possible direction ; so that 
whenever we look at the atmosphere, some of these rays fall 
upon our eyes ; hence we see the air of a blue color. If the 
atmosphere did not reflect any rays, though the objects on the 
surface of the earth would be illumined, the skies would 
appear perfectly black. 

Caroline. Oh, how melancholy that would be ; and how 
pernicious to the sight, to be constantly viewing bright objects 
against a black sky. But what is the reason that bodies often 
change their color ; as leaves which wither in autumn, or a 
spot of ink which produces an iron-mould on linen ? 

Mrs. B. It arises from some chemical change, which 
takes place in the internal arrangement of the parts, by which 
they lose their tendency to reflect certain colors, and acquire 
the power of reflecting others. A withered leaf thus no lon- 
ger reflects the blue rays ; it appears, therefore, yellow, or 
has a slight tendency to reflect several rays which produce a 
dingy brown color. 

An ink-spot on linen at first absorbs all the rays ; but^ 
exposed to the air, it undergoes a chemical change, and the 
spot partially regains its tendency to reflect colors^ but with 



204 ON REFRACTION AND COLORS. 

a preference to reflect the yellow rays^ and such is the color 
of the iron-mould. 

Emilij, Bodies, then, far from being of the color which 
they appear to possess, are of that color which they have the 
greatest aversion to, w^hich they will not incorporate with, 
but reject and drive from them. 

Mrs, B, It certainly is so ; though I scarcely dare ven- 
ture to advance such an opinion whilst Caroline is contem- 
plating her beautiful rose. 

Caroline. My poor rose ! you are not satisfied with 
depriving it of color, but even make it have an aversion to it ; 
and I am unable to contradict you. 

Emily, Since dark bodies absorb more solar rays than 
light ones, the former should sooner be heated if exposed to 
the sun ? 

Mrs, B, And they are found by experience to be so. 
Have you never observed a black dress to be warmer than a 
white one ? 

Emily, Yes, and a white one more dazzling : the black 
is heated by absorbing the rays, the white dazzling by reflect- 
ing them. 

Caroline, And this was the reason that the brown paper 
was burnt in the focus of the lens, whilst the white paper 
exhibited the most luminous spot, but did not take fire. 

Mrs, B, It was so. It is now full time to conclude our 
lesson. At our next meeting, I shall give you ^ (description 
of the eye. 



^^•<' 



TLATE TTT 




t3STERSATI0N XVH. 



OPTICS. 

ON THE STRUCTURE OF THE EYE, AND OPTICAL 
INSTRUMENTS. 

Description of the Eye ; Of the Image on the Retina ; 
Refraction of the Humors of the Eye ; Of the Use of 
Spectacles; Of the Single Microscope ; Of the Double 
Microscope ; Of the Solar Microscope ; Magic LaU' 
thorn ; Refracting Telescope ; Refecting Telescope, 



MRS.B. 

The body of the eye is of a spherical form : (fig. 1. plate 
XXL) it has two membranous coverings ; the external one^ 
a a Oy is called the sclerotica; this has a projection in that 
part of the eye which is exposed to view, b b, which is called 
the cornea, because, when dried, it has nearly the consist- 
ence of very fine horn, and is sufficiently transparent for the 
light to obtain free passage through it. 

The second membrane which lines the cornea, and envel- 
opes the eye, is called the choroid, c c c ; this has an open- 
ing in front, just beneath the cornea, which forms the pupil, 
d 5, through which the rays of light pass into the eve. The 
pupil is surrounded by a colored border, called the" iris, e e, 
which by its muscular motion, always preserves the pupil of 
a circular form, whether it is expanded in the dark, or con- 
tracted by a strong light. This you will understand better 
by examining fig. 2. 

Emily. 1 did not know that the pupil was susceptible of 
varying its dimensions. 

Mrs, B. The construction of the eye is so admirable, that 
it is capable of adapting itself, more or less, to the circum- 
18 



20G OPTICS. 

stances in which it is placed. In a faint light the pupil di- 
lates so as to receive an additional quantity of rays, and in a 
strong light it contracts, in order to prevent the intensity of 
the light from injuring the optic nerve. Observe Emily's 
eyes, as she sits looking towards the windows : her pupils 
appear very small, and the iris large. Now, Emily, turn 
from the light, and cover your eyes with your hand, so as 
entirely to exclude it for a few moments. 

Caroline. How very much the pupils of her eyes are 
now enlarged, and the iris diminished. This is, no doubt, 
the reason why the eyes suffer pain, when from darkness 
they suddenly come into a strong light ; for the pupil being 
dilated, a quantity of rays must rush in before it has time to 
contract. 

Emily, And when we go from a strong light into obscu- 
rity, we at first imagine ourselves in total darkness ; for a 
sufficient number of rays cannot gain admittance into the 
contracted pupil, to enable us to distinguish objects : but in a 
iew minutes it dilates, and we clearly perceive objects which 
were before invisible. 

Mrs. B. It is just so. The choroid c c, is imbued with a 
black liquor which serves to absorb all the rays that are 
irregularly reflected, and to convert the body of the eye into 
a more perfect camera obscura. When the pupil is expand- 
ed to its utmost extent, it is capable of admitting ten times 
the quantity of light that it does when most contracted. In 
cats, and animals which are said to see in the dark, the power 
of dilatation and contraction of the pupil is still greater : it is 
computed that their pupils may receive one hundred times 
more light at one time than at another. 

Within these coverings of the eye-ball are contained three 
transparent substances, called humors. The first occupies 
the space immediately behind the cornea, and is called the 
aqueous humor, ff^ from its liquidity and its resemblance to 
w ater. Beyond this is situated the crystalline humor, g g, 
so called from its clearness and transparency : it has the 
form of a lens, and refracts the rays of light in a greater de- 
gree of perfection than any that have been constructed by- 
art : it is attached by two muscles, m m, to each side of the 
choroid. The back part of the eye, between the crystalline 
humor and the retina, is filled by the vitreous humor, h k, 
which derives its name from a resemblance it is supposed to 
bear to elass or vitrified substances. 



OPTICS. 207 

The membranous coverings of the eye are intended chiefly 
for the preservation of the retina, i ?, which is by far the most 
important part of the eye, as it is that which receives the 
impression of the objects of sight, and conveys it to the mind. 
The retina consists of an expansion of the optic nerve, of a 
most perfect whiteness : it proceeds from the brain, enters 
the eye, at w, on the side next the nose, and is finely spread 
over the interior surface of the choroid. 

The rays of light which enter the eye by the pupil are 
refracted by the several humors in their passage through them, 
and unite in a focus on the retina. 

Caroline. I do not understand the use of these refracting 
humors : the image of objects is represented in the camera 
obscura, wdthout any such assistance. 

IMrs, B. That is true ; but the reoresentation would be 
much more strong and distinct, if we enlarge the opening of 
the camera obscura, and received tlie rays into it through a 
lens. 

I have told you that rays proceed from bodies in all possi- 
bFe directions. We must, therefore, consider every part of 
an object which sends rays to our eyes, as points from which 
the rays divergfe, as from a centre. 

Emilf/. These divergent rays, issuing* from a single point, 
I believe you told us, were called a pencil of rays ? 

Mrs. B. Yes. Now, divergent rays, on entering the 
pupil, do not cross each other ; the pupil, however, is suffi- 
ciently large to admit a small pencil of them ; and these, if 
not refracted to a focus by the humors, would continue diverg- 
ing after they had passed the pupil, would fall dispersed upon 
the retina, and thus the image of a single point would be 
expanded over a large portion of the retina. The divergent 
rays from every other point of the object would be spread 
over a similar extent of space, and would interfere and be 
confounded with the first ; so that no distinct image could be 
formed, and the retina would represent total confusion both 
of figure and color. Fig. 3. represents two pencils of rays 
issuing from two points of the tree A B, and entering the 
pupil C, refracted by the crystalline humor D, and forming 
distinct images of the spot they proceed from, on the retina at 
a b. Fig. 4. differs from the preceding, merely from not 
being supplied with a lens ; in consequence of which the 
pencils of rays are not refracted to a focus, and no distinct 

na<re is formed on the retina. T have delineated only the 



208 OPTICS. 

rays issuing from two points of an object, and distinguished 
tlie two pencils in fig. 4. by describing one of them with dot- 
ted lines ; the interference of these two pencils of rays on the 
retina will enable you to form an idea of the confusion which 
would arise, from thousands and millions of points at the same 
instant pouring their divergent rays upon the retina. 

Emily. True ; but I do not yet well understand how the 
refracting humors remedy this imperfection. 

Mrs. B. The refraction of these several humors unite the 
w^hole of a pencil of rays, proceeding from any one point of an 
object, to a corresponding point on the retina, and the image 
is thus rendered distinct and strong. If you conceive, in fig. 
3., ev-ery point of the tree to send forth a pencil of rays simi- 
lar to those, A B, every part of the tree will be as accurately 
represented on the retina as the points a h. 

Emily. How admirably, how wonderfully, this is con« 
trived ! 

Caroline. But since the eye requires refracting humors in 
order to have a distinct representation formed on the retina, 
why is not the same refraction necessary for the image formed 
in the camera obscura. 

Mrs. B. Because the aperture through which we received 
the rays into the camera obscura is extremely small ; so that 
but very few of the rays diverging from a point gain admit- 
tance ; but we will now enlarge the aperture, and furnish it 
with a lens, and you will find the landscape to be more 
peifecrly represented. 

Caroline. How obscure a.id confused the image is now 
that you have enlarged the opening, without putting in the 
lens. 

Mrs. B. Such, or ver}^ similar would be the representation 
on the retina, unassisted by the refracting humors. But see 
what a difference is produced by the introduction of the lens, 
which collects each pencil of divergent rays into their several 
foci. 

Caroline. The alteration is wonderful : the representation 
is more clear, vivid, and beautiful than ever. 

Mrs. B. You will now be able to understand the nature 
of that imperfection of sight, which arises from the eyes being 
too prominent. In such cases, the crystalline humor, D, 
(fig. 5.) being extreme^ convex, refracts the rays too much, 
and collects a pencil, proceeding from the object A B, into a 
focus, F, before they reach the retina. From this focus tlie* 




Tiq. 4 



OPTICS. 209 

rays proceed diverging^ and consequently form a very confused 
image on the retina at a b. This is the defect of short-sighted 
people. 

Emily, I understand it perfectly. But why is this defect 
remedied by bringing the object nearer to the eye^ as we find 
to be the case with short-sighted people ? 

Mrs. B, The nearer you bring an object to your eye the 
more divergent the rays fall upon the crystalline humor, and 
they are consequently not so soon converged to a focus : this 
focus therefore, either falls upon the retina, or at least ap- 
proaches nearer to it, and the object is proportionally distinct, 
as in fig. 6. 

Emily. The nearer, then, you bring an object to a lens 
the further the image recedes behind it. 

Mrs. B. Certainly. But short-sighted persons have 
another resource for objects which they cannot approach to 
their eyes ; this is to place a concave lens, C D, (fig. 1. plate 
XXII.) before the eye, in order to increase the divergence of 
the rays. The effect of a concave lens is, you know, exactly 
the reverse of a convex one : it renders parallel rays divergent 
and those which are already divergent, still more so. By the 
assistance of such glasses, therefore, the rays from a distant 
object fall on the pupil, as divergent as those from a less dis- 
tant object ; and, with short-sighted people, they throw the 
image of a distant object back as far as the retina. 

Caroline. This is an excellent contrivance, indeed. 

Mrs. B. And tell me, what remedy would you devise for 
such persons as have a contrary defect in their sight ; that is 
to say, in whom the crystalline humor, being too flat, does not 
refract the rays sufficiently, so that they reach the retina before 
they are converged to a point ? 

Caroline. I suppose that a contrary remedy must be ap- 
plied to this defect ; that is to say, a convex lens, L M, fig. 
2., to make up for the deficiency of convexity of the crystal- 
line humor, O P. For the convex lens would bring the rays 
nearer together, so that they would fall either less divergent, 
or parallel on the crystalline humor ; and, by being sooner 
converged to a focus, would fall on the retina. 

Mrs. B. Very well, Caroline. This is the reason why 
elderly people, the humors of whose eyes are decayed by age, 
are under the necessity of using convex spectacles. And 
when deprived of that resource, they hold the object at a 

18* 



;o OPTICS. 

distance from their eyes, as in fig, 4, in order to bring the locus 
forwarder. 

Caroline. I have often been surprized, when my grand- 
father reads without his spectacles, to see him hold the book 
at a considerable distance from his eyes. But I now under- 
stand it ; for the more distant the object is from the crystal- 
line, the nearer the image will be to it. 

Emily, I comprehend the nature of these two opposite 
defects very well ; but I cannot now conceive, how any sight 
can be perfect : for if the crystalline humor is of a proper 
degree of convexity, to bring the image of distant objects to a 
focus on the retina, it will not represent near objects, distinct- 
ly ; and if, on the contrary, it is adapted to give a clear image 
of near objects, it will produce a very imperfect one of distant 
objects. 

Mrs, B, Your observation is very good, Emily ; and it is> 
true, that every person would be subject to one of these two 
defects, if we had it not in our power to increase or diminish 
the convexity of the crystalline humor, and to project it 
towards, or draw it back from the object, as circumstances 
require. In a young well-constructed eye, the two muscles 
to which the crystalline humor is attached have so perfect a 
command over it, that the focus of the rays constantly falls on 
the retina, and an image is formed equally distinct both of 
distant objects and of those which are near. 

Caroline, In the eyes of fishes, which are the only eyes I 
have ever seen separate from the head, the cornea does not 
protrude, in that part of the eye which is exposed to view. 

Mrs, B, The cornea of the eye of a iish is not more convex 
ihan the rest of the ball of the eye ; but to supply this defi- 
ciency, their crystalline humor is spherical, and refracts the 
rays so much, that it does not require the assistance of the 
cornea to bring them to a focus on the retina. 

Emily, Pray, w^hat is the reason that we cannot see an 
object distinctly, if we approach it very near to the eye ? 

Mrs, B, Because the rays fall on the crystalline humor 
too divergent to be refracted to a focus on the retina ; the 
confusion, therefore, arising from viewing an object too near 
the eye, is similar to that which proceeds from a flattened 
(^rystalline humor ; the rays reach the retina before they are 
^^ollected to a focus, {Jig, 4.) If it were not for this imper- 
'■^<tion. we should be able to see and distinguish the parts of 



OPTICS. 2il 

objects^ which are now invisible to us from their minuteness ; 
for could we approach them very near the eye, their image 
on the retina would be so much magnified as to render them 
visible. 

Emily, And could there be no contrivance to convey the 
rays of objects viewed close to the eye, so that they should be 
refracted to a focus on the retina ? 

Mrs, B, The microscope is constructed for this purpose. 
The single microscope (fig. 5.) consists simply of a convex 
lens, commonly called a magnifying glass ; in the focus of 
which the object is placed, and through which it is viev/ed : 
by this means, you are enabled to approach your eye very 
near the object, for the lens A B, by diminishing the diver- 
gence of the rays, before they enter the pupil C, makes them 
fall parallel on the crystalline humor D, by which they are 
refracted to a focus on the retina, at R R. 

Emily, This is a most admirable invention, and nothing 
can be more simple, for the lens magnifies the object merely 
by allowing us to bring it nearer to the eye. 

Mrs, B, Those lenses, therefore, which have the shortest 
focus will magnify the object most, because they enable us to 
bring the object nearest to the eye. 

Emily, But a lens, that has the shortest focus, is most 
bulging or convex ; and the protuberance of the lens will 
prevent the eye from approachhig very near to the object. 

Mrs, B, This is remedied by making the lens extremel}^ 
small : it may then be spherical without occupying much 
space, and thus unite the advantages of a short focus, and of 
allowing the eye to approach the object. 

Caroline, We have a microscope at home, which is a 
much more complicated instrument than that you liave des- 
cribed. 

Mrs. B, It is a double microscope (fig. 6.), in which you 
see not the object A B, but a magnified image of it, a b. In 
this microscope, two lenses are employed, the one L M, for 
the purpose of magnifying the object, is called the object 
glass ; the other N O, acts on the principle of the single mi- 
croscope, and is called the eye-glass. 

There is another kind of microscope, called the solar mi- 
croscope, which is the most wonderful from its great magni- 
fying power ; in this we also view an image formed by a 
lens, not the object itself. As the sun shines, I can show you 
die effect of this microscope : but for this purpose, we must 



212 OPTICS. 

close the shutters, and admit only a small portion of light, 
through the hole in the window-shutter, which we used for 
the camera obscura. We shall now place the object A B, 
(plate XXIII. fig, 1.) which is a small insect, before the lens 
C D, and nearly at its focus ; the image E F, will then be 
represented on the opposite wall in the same manner as the 
landscape was in the camera obscura ; with this difference, 
that it will be magnified, instead of being diminished. I 
shall leave you to account for this, by examining the figure. 

Emily, 1 see it at once. The image E F is magnified, 
because it is farther from the lens, than the object A B ; 
while the representation of the landscape was diminished 
because it was nearer the lens, than the landscape was. A 
lens, then, answers the purpose equally well, either for mag- 
nifying or diminishing objects ? 

Mrs, B, Yes : if you wish to magnify the image, you 
place the object near the focus of the lens ; if you wish to 
produce a diminished image, you place the object at a dis- 
tance from the lens, in order that the image may be formed 
in, or near the focus. 

Caroline. The magnifying power of this microscope, is 
prodigious, but the indistinctness of the image for want of 
light, is a great imperfection. AVould it not be clearer, if the 
opening in the shutter were enlarged, so as to admit more 
light. 

Mrs, B, If the whole of the light admitted does not fall 
upon the object, the effect will only be to make the room 
lighter, and the image consequently less distinct. 

Emily, But could you not by means of another lens bring 
a large pencil of rays to a focus on the object, and thus con- 
centrate the whole of the light admitted upon it ? 

Mrs, B, Very well. We shall enlarge the opening and 
place the lens X Y (fig, 2.) in it, to converge the rays to a 
focus on the object A B. There is but one thing more wan- 
ting to complete the solar micr-oscope, which I shall leave to 
Caroline's sagacity to discover. 

Caroline, Our microscope has a small mirror attached to 
it, upon a moveable joint, which can be so adjusted as to 
receive the sun^s rays, and reflect them upon the object ; if a 
similar mirror were placed to reflect light upon the lens, 
would it not be a means of illuminating the object more 
perfectly. 

Mrs, B. You are quite right. P Q (fig. 2.) is a small 



j'LATi: xxm. 




c ^e- * 



OPTICS. 213 

mirror placed on the outside of the window shutter^ which 
receives the incident rays S S, and reflects them on the lens 
X Y. Now that we have completed the apparatus let us 
examine the mites on this piece of cheese^ which I place near 
the focus of the lens. 

Caroline. Oh, how much more distinct the image now is, 
and how wonderfully magnified ; the mites on the cheese look 
Uke a drove of pigs scrambling over rocks ? 

Emily, I never saw any thing so curious. Now, an 
immense piece of cheese has fallen : one would imagine it 
an earthquake : some of the poor mites must have been 
crushed ; how fast they run, — they absolutely seem to gallop. 

But this microscope can be used only for transparent ob- 
jects ; as the light must pass through them to form the image 
on the wall. 

Mrs. B. Very minute objects, such as are viewed in a 
microscope, are generally transparent ; but when opaque ob- 
jects are to be exhibited, a mirror M N (fig. 3.) is used to 
reflect the light on the side of the object next the wall : the 
image is then formed by light reflected from the object, instead 
of being transmitted through it. 

Einiiy, Pray is not a magic lanthorn constructed on the 
same principles ?'* 

Mrs. B* Yes ; with this difference that the light is sup- 
plied by a lamp, instead of the sun. 

The micro.'^cope is an excellent invention, to enable us to 
see and distinguish objects, which are too small to be visible to 
the naked eye. But there are objects which, though not 
really small, appear so to us, fi'om their distance ; to these we 
cannot apply the same remedy ; for when a house is so far 
distant, as to be seen under the same angle as a mite which is 
close to us, the effect produced on the retina is the same : the 
angle it subtends is not large enough for it to form a distinct 
image on the retina. 

Emily. Since it is impossible, in this case, to approach the 
object to the eye, cannot v/e by means of a lens bring an image 
of it nearer to us ? 

Mrs. B. Yes ; but then the object being very distant 
from the focus of the lens, the image weald be too small to be 
visible to the naked eye. 

* The ma£^ic lanthorn is an instrument used for magnifying paintings on 
glass, Riu! throwing- their images upon a white screen in a darkene(t 
uhambi^r. 



J 14 OPTICS. 

Emily, Then, why not look at the image through another 
lens, which will act as a microscope, enable us to bring the 
image close to the eye, and thus render it visible ? 

Mrs, B. Very well, Emily ; I congratulate you on hav- 
ing invented a telescope. In figure 4, the lens C D, forms an 
image E F, of the object A B ; and the lens X Y, serves the 
purpose of magnifying that image ; and this is all that is 
required in a common refracting telescope. 

Emily. But in fig. 4, the image is not inverted on the 
retina, as objects usually are : it should therefore appear to 
us inverted ; and that is not the case in the telescopes I have 
looked through. 

Mrs, B. When it is necessary to represent the image 
erect, two other lenses are required ; by which means a second 
image is formed, the reverse of the first and consequently 
upright. These additional glasses are used to view terres- 
trial objects ; for no inconvenience arises from seeing the 
celestial bodies inverted. 

Emily. The difference between a microscope and a teles- 
cope seems to be this : — a microscope produces a magnified 
image, because the object is nearest the lens ; ai d a telescope 
produces a diminished image, because the object is furthest 
from the lens. 

Mrs. B. Your observation applies only to the lens C D, 
or object glass, which serves to bring an image of the object 
nearer the eye ; for the lens X Y, or eye-glass is, in fact, a 
microscope, as its purpose is to magnify the image. 

When a very great magnifying power is required, teles- 
copes are constructed with concave mirrors, instead of lenses. 
Concave mirrors, you know, produce by reflection, an effect 
similar to that of convex lenses by refraction. In reflecting 
telescopes, therefore, mirrors are used in order to bring the. 
image nearer the eye ; and a lens or eye-glass the same as in 
tJie refracting telescope to magnify tlie image. 

The advantage of the reflecting telescope is, that mirrors 
whose focus is six feet will magnify as much as lenses of a 
hundred feet. 

Caroline. But I thought it was the eye-glass only which 
magnified the image ; and that the other lens served to bring 
a diminished image nearer to the eye. 

Mrs. B. The image is diminished in comparison to the 
object, it is true ; but it is magnified if you compare it to the 
dimensions of which it would appear without the intervention 



OPTICS. 215 

of any optical instrument ; and this magnifying power is 
greater in reflecting than in refracting telescopes. 

We must now bring our observations to a conclusion, for I 
have communicated to you the whole of my very limited 
stock of knowledge of Natural Philosophy. If it will enable 
you to make further progress in that science, my wishes will 
be satisfied ; but remember that, in order that the study of 
nature may be productive of happiness, it must lead to an 
entire confidence in the wisdom and goodness of its bounteous 
Author. 



FOR THE 



EXAMI^ATIOJ^ OF SCHOULRS. 



QUESTIONS TO CONVERSATION L 



ON THE GENERAL PROPERTIES OF BODIES. 

Introduction ; General Properties of Bodies ; Impenetra- 
hility ; Extension ; Figure ; Divisibility ; Inertia ; 
Attraction ; Attraction of Cohesion ; Density ; Rarity ; 
Heat ; Attraction of Gravitation* 



1. What is to be understood by the term bodies, as used 
m philosophy ? 

2. What term is used to denote substances ? 

S. What properties are common to all bodies ? 

4. Why are these called general properties of bodies r 

5. What is impenetrability ? 

6. Can liquids occupy the same space of a solid body ? 

7. How can you prove that liquids cannot occupy the 
same space occupied by solids ? 

8. Can liquids and air occupy the same space in the same 
time ? 

9. How can you prove that they cannot ? 

10. What is extension ? 

11. What are the dimensions of a body ? 

12. What is the difference between height and depth, as 
applied to extension ? 

19 



218 GENERAL PROPERTIES OF BODIES. 

13. What is the figure of a body ? 

14. What is divisibility in natural philosophy ? 

15. What are instances of practical divisibility of matter' 
to a great extent ? 

16. On what principle is it that we can smell different 
odoriferous objects ? 

17. If we inhale particles of odoriferous objects, why can 
we not see these particles ? 

18. If the particles of a phial of fragrant liquid escape 
from the liquid in order to perfume a room, does the liquid 
suffer any diminution ; and if so, why can we not perceive it, 
when it takes place ? 

19. On what principle are wood and other substances 
burnt, as it is termed, when commhted to the fire ? 

20. Is not the matter of which wood is composed destroy- 
ed, when burnt to ashes ; and if not, wliy can we not see a 
greater part of the disunited particles ? 

21. Is it then a principle in philosophy that there has been 
and can be no diminution of matter — not of a single particle ? 

22. What is inertia ? 

23. Will bodies always remain at rest, unless an external 
force is applied to them ? 

24. And what would be the consequence if a body were 
put in motion and no resistance should be offered ? 

25. What is attraction? 

26. Why is this property of matter called the attraction of 
cohesion ? 

27. What would be the consequence if the power of the 
attraction of cohesion were destroyed ? 

28. Does the attraction of cohesion exist also in liquids ? 

29. How can you prove that it exists in liquids ? 

30. Why are some bodies hard and others soft ? 

31. To what is the cohesive attraction in liquids propor- 
tioned ? 

32. Does the attraction of cohesion exist in the air ? 

33. But are the particles of the air actually under the 
influence of this attraction ? 

34. Why are they not, if attraction belongs to them ? 

35. How do we know that attraction does belong to the 
air, if no influence is exerted upon it ? 

36. Why is it that some liquids are thick and others thin ? 

37. What is density ? 

38. What is rarity ? 



GENERAL PROPERTIES OF BODIES. 219 

39. How are we to judge of the quantity of matter in bodies ? 

40. In what proportion are bodies of the same bulk dense ? 

41. What bodies are said to be dense ? 

42. What bodies are said to be rare ? 

43. Why are not sponge and cork and other similar sub- 
stances hard since their particles come in contact ? 

44. What fluid is named as more subtle than air ? 

45. What effect has heat on bodies ? 

46. What two forces are said to act always on bodies in 
opposition to each other ? 

47. In what cases may we see the effect of heat in the 
expansion of bodies, or in the separation of their particles ? 

48. How are liquids made to boil by heat ; or how is the 
motion or agitation of boiling liquids produced ? 

49. Why are one^s hands and fingers swollen or larger 
on being held near thefire^ than ivhen exposed to the cold? 

50. Why is water collected in drops on leaves after a rain ? 

51. Does rain leave the clouds in the form of drops, as 
they reach the earth ? 

52. How then do these drops become formed ? 

53. Whence does the dew collect itself into drops ? 

54. What causes the rain or water to fall from the clouds ? 

55. What causes water to rise in a capillary tube, above its 
level without the tube ? 

56. What causes water to rise in sponge and other porous 
substances above its level ? 

57 » If several tubes of different bore are immersed in water^ 
in which will it rise highest ? 

58. What is gravitation ? 

59. What is the difference between cohesive attraction and 
gravitation ? 

60. What causes bodies to fall to the earth ? 

61. In what proportion do bodies attract or gravitate 
towards each other ? 

62. What would be the consequence of gravitation on 
bodies, were it not for cohesive attraction ? 

63. W^hat is the reason that cohesive attraction does not 
operate on different bodies brought into contact as well as on 
the particles of the same body ? 

64. When will the surfaces of different bodies adhere to 
nch other by the force of cohesive attraction ? 



QUESTIONS TO CONVERSATION IL 



ON THE ATTRACTION OF GRAVITY. 

Attraction of Gravitation^ continued ; Of weight ; Of the 
fall of Bodies ; Of the resistance of the Air ; Of the 
Ascent of Light Bodies, 



65. AVhat are general or common properties of bodies ? 

66. What are the accidental properties of bodies ? 

67. Are color and weight general or accidental properties ? 
6^. What is weight, or of what is it the effect ? 

69. If bodies mutually attract each other, why is not the 
earth drawn to other bodies, as well as they drawn to the 
earth ? 

70. If there were but one body in the universe, would 
there be any such thing as weight ? 

71. Can cohesive attraction exist where there is no weight ? 

72. If the earth attracts all objects to it, why are not 
houses and other objects at the side of a mountain attracted 
or drawn away from their foundations ? 

7S. Do hills and mountains possess a sideways attraction ? 

74. How can it be proved ? 

75. Would two lines suspended by weights be parallel to 
each other ? 

76. Wh}^ would they not be ? 

77' If they are not parallel, v/hy do we not perceive their 
convergency ? 

78. What is the object of the Figure 1, Plate I. ? 

79. Do heavy and light bodies fall to the ground with 
equal rapidity ? 

80. Which fall with the greater rapidity ? 

81. Why do heavy bodies fall quicker than lighter ones ? 

82. To w4iat is the resistance of the air to falling bodies 
proportioned ? 

83. Do large and small bodies require the same degree of 
attraction to bnng them to the ground in the same time ? 



ON THE ATTRACTION OF GRAVITY. 221 

84. Which require the greater degree of attraction ? 

85. How can a heavy body be made to float in the air 
instead of falling immediately to the ground ? 

86. Does the air gravitate towards the earth ? 

87. If then the air gravitates towards the earth, why does 
it not fall or settle completely to the surface of the earth ? 

88. What two forces continually operate against each 
other on the air ? 

89. Is the air of the same density at the surface of the earth 
as at a distance from it ? 

90. At which is the density the greatest ? 

91. Why is the air more dense at the surface of the earth 
than at a distance from it ? 

92. To what has the pressure of the atmosphere been 
compared ? 

93. What bodies do not gravitate towards the earth ? 

94. How does gravity operate in causing smoke and steam 
to ascend ? 

95 ♦ How high will smoke and steam rise before they re- 
main stationary ? 

96. Why will paper rise upon the top of water instead of 
sinking to the bottom like a stone ? 

97« On what principle does a balloon rise, since it is made 
of materials heavier than the air through which it rises ? 

98. How is the gravity of bodies modified by the eflect 
0^ the air ? 

99. Can a feather be placed in a situation to fall as quick or 
as heavy as stone ? 

100. How can it be done ? 



19* 



QUESTIONS TO CONVERSATION III. 



ON THE LAWS OF MOTION. 

On Motion ; Of the Inertia of Bodies ; Of Force to pro- 
duce Motion ; Direction of Motion ; Velocity ^ Absolute 
and Relative ; Uniform Motion ; Retarded Motion ; 
Accelerated Motion ; Velocity of Falling Bodies ; 
Momentum ; Action and Re-action Equal ; Elasticity of 
Bodies; F or osity of Bodies ; Reflected Motion ; Angle^i 
of Incidence and Reflection, 



101. On what is the science of mechanics founded ? 

102. What is motion ? 

103. Can a body move itself ? 

104. What is the power called which puts a body in 
motion ? 

105. What is it that binds the particles of a body together ? 

106. What forces them asunder ? 

107. In what direction is the motion of a body acted on by 
a single force ? 

108. What is the velocity of motion ? 

109. To what is velocity proportioned ? 

110. What is absolute velocity ? 

111. What is relative velocity ? 

112. What is uniform motion ? 

113. What is accelerated motion ? 

114. What is retarded motion ? 

115. What are instances of accelerated motion ? 

116. What are instances of retarded motion ? 

117. How far will a heavy body, suspended in air, fall the 
lirst second of time ? 

118. How far the second ? 

119. How far the third second ? 

120. How does the time of an ascending body thrown 
into the air, always compare with the time of its descent ? 

121. What is the momentum of a body ? 



ON THE LAWS OF MOTION. 223 

122. In what way can a small body have a greater mo 
mentum than a large one ? 

123. What is the re-action of a body ? 

124. To what is the re-action of a body equal ? 

125. What is the object of Figure 3, Plate I. ? 

126. How would you explain that figure ? 

127. How would you explain the Figure 4, in Plate I. ? 

128. Is the re-action of all bodies equal to the action, 
when a blow is given ? 

129. In what bodies is it equal ? 

130. In w^hat ones is it not equal ? 

131. What is the object of Figure 5, Plate I. ? 

132. How will you explain it ? 

133. On what principle is it that birds are able to fly ? 

134. How must a bird strike the air with its wings, so as to 
remain stationary ? 

135. How so as to rise ? 

136. How so as to descend ? 

137. Why will a bird remain longer in the air with its 
vv^ings extended than when they are closed j although they are 
not moved ? 

138. If flying is only the effect of a re-action, why could not 
a man be furnished with wings so as to fly ? 

139. How is swimming effected ? 

140. On what principle and how is a boat moved on the 
water ? 

141. What bodies besides air are elastic ? 

142. What bodies are not elastic ? 

143. What is it that produces the elasticity of bodies ? 

144. Is it supposed that ivory, metals, and other hard 
bodies are porous ? 

145. What conjecture did Sir Isaac Newton form concer- 
ning the porosity of the earth ? 

1 46. What is reflected motion ? 

147. If a ball is thrown against a wall, in what direction is 
the reflected motion ? 

148. What is a perpendicular direction ? 

149. What is an angle ? 

150. What is a right angle ? 

151. What is the object of Figure 1, Plate II. ? 

152. How are all circles supposed to be divided ? 

153. How many degrees are contained in the two angles 
formed by the Figure named ? 



224 ON COMPOUND MOTIOX. 

154. What is an obtuse angle ? 

155. How would you explain Figure 4, Plate II. : 

156. What is an angle of incidence ? 
1 57- What is an angle of reflection ? 

158. How does the angle of incidence compare with the 
angle of reflection as to size ? 



QUESTIONS TO CONVERSATION IV. 



ON COMPOUND MOTION. 

Compound Motion^ the result of two Opposite Forces ; Of 
Circular Motion, the Result of two Forces one of which 
confines the Body to a Fixed Point ; Centre of Motion, 
the Point at Rest while the other parts of the Body move 
round it ; Centre of Magnitude , the Middle of a Body ;. 
Centripetcd Force, that zvhich confines a Body to a fixed 
Central Point ; Centrifugal Force, that ivhich impels a 
Body to fly from the Centre ; Fall of Bodies in a Para- 
bola ; Centre of Gravity, the Centre of Weight, or point 
about which the Parts balance each other. 



150. What is compound motion ? 

160. How would a body move struck by two equal forces 
in opposite directions ? 

161. What is the object of Figure 5, Plate n. ? 

162. How would you explain that figure? 

163. What is the design of Figure 6, Plate II. ? 

164. How would you explain it ? 

165. What is the line A. D. called in the Fisfure 5 of Plate 
II.? 

166. What is the line A. D. called in Figure 6, Plate 11. ? 

167. What are the lines A. D. and B. C. called in Figure 
7, Plate II. ? 

168. Of what is circular motion the result ? 

169. What instance of circular motion thus produced could 
you give ? 



ON COMPOUND MOTION. 225 

170. What is the axis of motion ? 

171. Is the velocity of motion the same at a distance from 
as near the centre of motion ? 

172. What is the object of Figure 1, Plate III. ? 

173. What are the forces called in circular motion^ that 
balance or act in opposition to each other ? 

174. What is centripetal motion ? 

175. What is centrifugal motion ? 

176. What would be the consequence if in circular motion 
the centripetal should be destroyed ? 

177. What is a tangent ? 

178. How would you describe Figure 2 in Plate III. ? 

179. If curved lines or circular motion are produced by 
the operation of two forces, how is it, that the falling of a 
stone thrown into the air is in a circular direction ? 

180. What is a parabola ? 

181. Why will a stone thrown perpendicularly into the air 
descend perpendicularly ? 

182. What is the centre of gravity ? 

183. What part of a body must be supported to keep the 
body from falling. 

184. What would be the consequence if the centre of 
gravity were not supported ? 

185. What causes or when will a loaded carriage be turned 
over or upset ? 

186. How would you explain Figure 4 in Plate III. ? 
187- How is it that rope dancers are able to perform their 

feats of agility without falling ? 

188. W'hy do persons in ascending^ a hill incline or stoop 
forward, and in descending a hill incline backwards ? 

189. How would you explain Figure 5 in Plate III. ? 

190. Is the centre of gravity always in the middle of a body ? 

191. When is the centre of magnitude the centre of gravity ? 

192. What bodies are the most easily upset ? 

193. Why can a person carry two pails of water, one in 
each hand, easier than one pail ? 

194. How would you explain Figure 6, Plate III. ? 

195. Where must the line falling perpendicularly from the 
centre of gravity in a wine glass strike the table on which it 
stands so as not to be turned over ? 

196. How are two bodies connected together to be con- 
sidered as to their centre of gravity ? 

1 97- How are Figui^s 7, 8 and 9 in Plate III. to be explained ? 



QUESTIONS TO CONVERSATION V. 



OjY the mechanical powers. 

Of the Foicer of Machines ; Of the Lever in General ; Of 
the Lever of the First Kind., having the Fulcrum between 
the Power and the Weight ; Of the Lever of the Second 
Kind, having the Weight between the Power and the 
Fulcrum; Of the Lever of the Third Kind y having the 
Power betiveen the Fulcrum and the Weight. 



198. How many of the mechanical powers are there ? 

199. What are they called ? 

200. In order to understand the power of a. machine, how 
many things are to be considered ? 

201. What is the first .^ 

202. What is the second ? 

203. What is the third ? 

204. What is the fourth ? 

205. What is the lever in mechanics ? 

206. What is the fulcrum in a pair of scales ? 

207. Why are the scales in F ig. 1 , Plate IV. in equilibrium ? 

208. What is the centre of gravity to two scales in equili- 
brium ? 

209. In what way can a level be used so that bodies of 
different weights may balance each other ? 

210. What are the arms of a lever ? 

211. Why are not the two arms of a lever in equilibrium, 
though of unequal length ? 

212. How can the two arms of a lever be brought intQ, 
equilibrium ? 

213. What is the design of Figure 4^ Plate IV. ? 

214. How many kinds of levers are there ? 

215. Where is the fulcrum in the first kind ? 

216. How are we to use the lever in raising or lifting large 
weights ? 

217. What power of mechanics do scissors involve ? 



ON THE MECHANICAL POWERS, 22^ 

218. How may the scissors be explained as formed by the 
lever ? 

219. How is the second kind of levers designed ? 

220. What are the most common examples of levers of 
the second kind, or where the weight is placed between the 
fulcrum and the power ? 

221. How would you explain the opening of a common 
door, as involving the principle of the second kind of levers ? 

222. What is the third kind of levers ? 

223. What is an instance of its use ? 

224. How does the raising a weight by the hand represent 
this kind of levers ? 



QUESTIONS TO CONVERSATION V, 
CONTINUED. 

ON THE MECHANICAL POWERS. 

Of the Pulley ; Of the Wheel and Axle ; Of the Inclined 
Plane ; Of the Wedge ; Of the Screw, 



k 



225. What is the second mechanical power ? 

226. What is a pulley ? 

227. What is the fulcrum of a pulley ? 

228. What is the design of Figure 1, Plate V. ? 

229. How would you explain Figure 2, in Plate V. ? 

230. In what does the advantage of a moveable ^nWey 
consist ? 

231. How do the weight and power of a moveable pulley 
compare, that their momenta be equal ? 

232. In the use of the moveable pulley is there no loss of 
time ? 

233. And to what is the loss of time proportioned ? 

234. What then is the advantage of this pulley, if there is 
as much loss in time as gain in power ? 

235. If there is no gain in time or power from the use of 
the fixed pulley, why is it used ? ^ 



228 ON THE MECHANICAL POWERS. 

236. What is the third mechanical power .^ 

237. What is the design of Figure 5, Plate V. ? 

238. In what proportion is the power of the whe^l 
increased .^ 

239. How may a wheel be compared to the lever .^ 

240. How would you explain Figure 6 in Plate. V. ? 

241. On what mechanical force is the windmill operated ? 

242. What is the fourth mechanical power ? 

243. What is an incHned plane ? 

244. How would you explain Figure 6, Plate V. ? 

245. What is the fifth mechanical power? 

246. Of what is the wedge composed ? . 

247. In what does the resistance to the wedge consist ? 
24S . On w^hat mechanical principle are cutting instruments 

designed ? 

249. What is the last mechanical power ? 

250. To which of the other mechanical powers is the 
screw referable ? 

251. What diminishes the force of all machinery ? 

252. What do we understand by friction in machinery ? 

253. In what proportion is the friction of machinery 
di ninished ? 

254. What is the reason for putting oil or grease upon the 
axles of wl ee s and on other machinery ? 

255. H« w many kinds of friction are there ? 

256. What are they .^ 

257. Which will the most readily overcome obstacles, a 
large or small wheel ? 

258. Why is a wheel fastened on descending a hill ? 

259. What besides friction diminishes the force of ma- 
chinery ? 

260. In what state would the force of machinery be perfect-^ 



QUESTIONS TO CONVERSATION VL 



CAUSES OF THE EARTH'S ANNUAL MOTION. 

Of the Planets, and tlieii^ Motion ; Of the Diurnal Motion 
of the Earth and Planets, 



261. If bodies attract each other in proportion to the 
quantity of matter, why does not the sun attract the earth 
completely to itself.^ 

262. If the earth on its creation had been put in motion by 
a single force without any resistances, what would have been 
its course ? 

263. How would you explain Figure 1, Plate VI. ? 

264. In that figure in what direction does the attraction of 
the sun operate on the projectile force of the earth ? 

265. When two forces operate perpendicularly on each 
other, in what direction will be their compound motion ? 

266. Why then is the line A. D. in Figure 1, circular 
instead of a right line diagonal to the parallelogram, A B 

267. How would you explain that part of Figure 1 , mark- 
ed D F G E ? 

268. And that part marked G H I K ? 

269. What is the centripetal force of the earth ? 

270. What is its centrifugal force ? 

271. How would you explain Figure 2, Plate VI. ? 

272. In Figure 3, Plate VI, why is the earth carried in 
the line A B instead of the line A C according to the princi- 
ple of Figure 1 ? 

273. When the earth arrives at E, why does it not move 
round the sun in a small orbit instead of receding off from the 
sun as at G ? 

274. What is the object of Figure 4, Plate VI. ? 

275. What is that part of the earth's orbit called farthest 
from the sun ? ^ 

276. What is that nearest the sun ? 

20 



230 ON THE PLANETS. 

277. How much nearer is the earth to the sun in its parhe- 
lion than at its aphelion. 

278. Is the earth nearest the sun in summer or winter ? 

279. How can we account for its being coldest when near- 
est the sun ? 

280. How much longer is the earth performing the sum« 
mer half than the winter half of its orbit ? 

281. What are the planets ? 

282. If they are worlds like our own, why do they appear 
so small ? 

283. If the fixed stars are suns, with planets revolving 
round them, why should we not see those planets as well a§ 
their suns ? 

284. Why do we not see the stars in the day time ? 

285. If the planets are only seen by the reflected light of 
the sun, how is it that they can be seen in the night ? 

286. What motion have the planets besides that about the 
sun.^ 



QUESTIONS TO CONVERSATION VH. 



ON THE PLANETS. 

Of the Satellites or Moons ; Gravity diminishes as the 
Square of the distance ; Of the Solar System ; Of 
Comets ; Constellations^ Signs of the Zodiac ; Of 
Copernicus^ Newton^ Sfc, 



287. How are the planets distinguished ? 

288. Which are the primary planets ? 

289. Which are the secondary ? 

290. By what names are the secondary planets called ? 

291. Why does not the sun attract the secondary planets 
from their primaries ? 

292. To what is the force of attraction proportioned besides 
the quantity of matter in the attracting bodies ? 



ON THE PLANETS. 23/ 

293. In what proportion is force of attraction diminished 
by distance ? 

294. What motion has the earth besides that about the sun 
and on its own axis ? 

295. Do the planets revolve round the centre of the sun ? 

296. Around what point do they revolve .^ 

297. Has the sun any motion f 

298. How is it known, that the sun turns on its own axis ? 

299. What motion has the sun besides that around its axis ? 

300. Do the planets at different distances revolve round 
the sun in the same time ? 

301. Why do the more distant from the sun revolve slower 
than those nearer to it ? 

302. What is the object of Figure 1, Plate VII. ? 

303. What is the object of Figure 2 of that plate ? 

304. How far from the sun is Mercury ? 

305. In what time does it revolve round the sun ? 

306. How far is Venus from the sun ? 

307. In what time does it revolve round the sun ? 

308. By what names do we call Venus .^ 

309. How far is the earth from the sun ? 

310. In what time does it revolve round the sun 7 

311. Which planet is next to the earth in distance from 
the sun ? 

312. How far is Mars from the sun ? 

313. How long time is occupied in his revolution about 
the sun ? 

314. What four small planets are next to Mars in distance 
from the sun ? 

315. AVhat did Dr. Herschel call these planets? 

316. What is the distance of Jupiter from the sun 

317. In what time is its revolution performed } 

318. How much larger is Jupiter than our earth ? 

319. How many moons or satellites has Jupiter ? 

320. Which planet is next in order ? 

321. How far is Saturn from the sun ? 

322. In what time does it revolve round the sun ? 

323. How many moons has Saturn ? 

324. How is Saturn distinguished from the other planets ? 

325. How many moons has Herschel or the Georgium 
Sidus ? 

326. How much more light and heat have we than Sa- 
turn ? 



232 ON THE PLANETS. 

327. Are comets supposed to be planets ? 

328. What are the constellations ? 

329. What are their names ? 

330. How would you explain Figure 1, Plate VIII. ? 

331. What is to be understood by the signs or constella- 
tions being in the zodiac ? 

332. On what is the different size and brilliancy of the 
fixed stars depending ? 

333. How may a fixed star he easily distinguished from 
a planet ? 

334. By ivhat is the twinkling light of the fixed stars 
occasioned ? 

335. If the earth is continually revolving on its axis, wh}/ 
do we not perceive its motion ? 

336. In case the earth thus revolves every 24 hours, do 
not the Sun and stars appear to us as if they revolved about 
the earth ? 

337. How would you illustrate this ? 

338. Why is it more probable the earth revolves than that 
the Sun and stars do ? 

339. How fast would a person move in the latitude of 
London by the motion of the earth upon its axis ? 

340. How fast does the earth move about the Sun ? 

341. What was the system of Ptolemy concerning as- 
tronomy ? 

342. What is the present system of astronomy called ? 

343. Who was the founder of the present system of as- 
tronomy ? 

344. When was the Copernican system of astronomy 
adopted ? 

345. What important discovery did Newton make in regard 
io the Copernican system ? 

346. What led Newton to his discoveries ? 

347. Hjw much greater is the diameter of the sun than of 
the earth ? 

348. HoiD much does its cubit magnitude exceed that of the 
earth ? 

349. Hitc is it known that the sun revolves upon its axis ? 

350. What does Dr, Herschel suppose the dark spots on 
the sun^s disk to be ? 



QUESTIONS TO CONVERSATION VIIL 



ON THE EARTH. 

On the Terrestrial Globe ; Of the Figure of the Earth ; Of 
the Pendulum ; Of the Variation of the Seasons, and of 
the Length of Days and Nights ; Of the causes of the 
Heat of Summer ; Of Solar, Siderialy and Equal or 
Mean Time. 



351. How does it appear that the earth is of a globular 
form ? 

352. What is the axis of the earth ? 

353. What are the poles ? 

354. What is the equator ? 

355. What is the ecliptic ? 

356. What is the earth's orbit? 

357' If the ecliptic relates only to the heavens, why is it 
described on the terrestrial globe ? 

358. What does Figure 1, Plate IX. represent ? 

359. What are the zones ? 

360. What is the torrid zone ? 

361. Where are the temperate zones ? 

362. Where are the frigid zones ? 

363. What are the meridian lines ? 

364. When will it be twelve o'clock at noon to all places 
under any particular meridian ? 

365. What circles are called greater circles ? 

366. What ones are called lesser circles ? 

367. Into how many parts are circles divided ? 

3 68. What is the diameter of a circle ? 

369. How many degrees does the diameter of a circle 
contain ? 

370. How many degrees are there in a meridian reaching 
from one pole to the other ? 

371. How many degrees are there between the equator 
and the poles ? 

20* 



234 ON THE EARTH. 

372. Are the degrees of longitude in different latitudes ol 
the same length ? 

37s. What is the length of a degree of latitude ? 

374. What is the reason that a degree of longitude on the 
equator is not the same as a degree of latitude ? 

37 o. Why is the earth supposed to be protuberant at the 
equator ? 

376, Why does the head of a person in the motion occa- 
sioned by the revolution of the earth on its axis move faster 
than his feet ? 

377' W^hat is a sphere or globe ? 

378. Will the same body weigh the same at the equator as 
at the poles ? 

379- At which is it the heaviest ? 

380. Why is it heaviest at the poles ? 

381. Were one to penetrate deep into the earth, would 
the force of gravity increase ? 

382. Why not ? 

383. Has an attempt ever been made to ascertain whether 
bodies will weigh heavier at the poles than at the equator ? 

384. By whom ? 

385. Could the experiment be made by the common 
scales ? 

386. Why not ? 

387- By what was it made ? 

388. Why does a pendulum vibrate ? 

389. Why are not its vibrations perpetual ? 

390. How can a pendulum determine whether objects are 
heavier at the poles or at the equator ? 

391. Why will a pendulum vibrate slower by increasing 
its length ? 

392. Hoiv do the pendulums used at the equator and at 
•polar regions compare in length ? 

393. IVhy does a clock go faster in lointer than in 
summer ? 

394. What causes the variation of seasons^ and the change 
of day and night ? 

395. How much is the axis of the earth inclined to the 
plane of the ecliptic ? 

396. What are nodes ? 

397. How would you explain Figure 2 in Plate IX. ? 

398. Why in the polar regions is it six months day and six 
months night ? 



ON THE EARTH* 235 

399. Why are the points where the ecliptic cuts the equa- 
tor called the equinoxes ? 

400. What point in ecliptic is called the summer solstice ? 

401. What is the winter solstice ? 

402. Why is the heat greater at the equator than at the 
distance from it ? 

403. Why is the heat of perpendicular rays more intense 
than that of oblique ones ? 

404. By which figure is this illustrated ? 

405. What is the object of figure 3, Plate X. ? 

406. What is the object of Figure 4, Plate X. .^ 

407. Wiiy does the Sun give more heat at mid-day, than 
in the morning and towards evening ? 

408. Why is it warmer in July and August than in June, 
when the days are the longest ; and at 2 and 3 P. M. than 
at noon ? 

409. Have the other planets the same vicissitudes of 
seasons ? 

410. How is it that the earth performs 366 complete rev- 
olutions in a 3^ear which has but 36j days and nights ? 

411. Why do the fixed stars appear to go round the earth 
quicker than the sun ? 

412. What is a siderial day ? 

413. What is a siderial year ? 

414. What is a solar day ? 

415. What is a solar year ? 

416. How can we know when the earth has performed 
one complete revolution about the Sun ? 

417. How would you explain Figure 1, Plate XI.? 



QUESTIONS TO CONVERSATION IX. 



ON THE MOON. 



Of the Moon^s Motion ; Phases of the Moon ; Eclipses of 
the Moon ; Eclipses of Jupiter^ s Moons ; Of the Latitude 
and Longitude ; Of the Transits of the Inferior Planets ; 
Of the tides. 



418. In what time does the moon revolve about the earth ? 

419. In what time does the moon revolve on its axis ? 

420. How is it known how long it takes the moon to 
revolve on its axis ? 

421. Does the earth exhibit the same changes to the moon, 
that the moon exhibits to the earth ? 

422. What are these changes called ? 

423. How would you explain Figure 2, Plate XI. ? 

424. When is the moon said to be in conjunction with the 
sun ? 

425. How is the moon eclipsed.^ 

426. How is the sun eclipsed ? 

427. At what time of the moon can she be eclipsed ? 

428. At what time of the moon can the sun be eclipsed ? 

429. As the moon passes between the sun and the earthy 
and as the earth passes between the sun and the moon, once 
every month, why do we not have a lunar and solar eclipse 
every month ? 

430. When does a partial eclipse take place ? 

431. What is the object of Figure 1, Plate XH. ? 

432. How can the comparative size of the earth and moon 
be determined by a lunar eclipse ? 

433. How would you explain Figure 2, Plate XII. ? 

434. When is the earth eclipsed to the moon ? 

435. Are the eclipses of the distant planets frequent ? 

436. What benefit do we derive from them ? 

437» How can the latitude of a place be determined ? 



ON THE MOON. 237 

438. How can the longitude of a place be determined by 
observation of the heavenly bodies ? 

439. How can it be determined at sea by the use of two 
watches ? 

440. What occasions the tides ? 

441. Why are the tides occasioned by the Moon^ unless 
water is acted on more powerfully by gravitation than the 
land ? 

442. If the tides are occasioned by the attraction of the 
moon, lohy is there a high tide on the side of the earth 
opposite the moon as ivell as on that next to it ? 

443 » How will you explain Figure 3, Plate XH. ? 

444. As the attraction of the Sun is greater than that of 
the moon, why does not the sun produce the chief influence 
in the tides ? 

445. But does the Sun exercise no influence in the produc- 
tion of the tides ? 

446. When does the sun increase the tides ? 

447. What is meant by the sun and moon acting in 
conjunction on the tides ? 

448. What are tides called produced by the conjunctive 
attraction of the sun and moon ? 

449. What are neap tides ? 

450. How would you explain Figure 4, Plate XII. ? 

451. How would you explain Figure 5, Plate XH. ? 

452. In what parts of the earth are tides the highest ? 

453. Why are they highest at the equator ? 

454. Why are the tides three quarters of an hour later 
5? very day ? 

455. Why are tides highest at the new and full of the moon ? 



QUESTIONS TO CONVERSATION X. 



ON THE MECHANICAL PROPERTIES OF FLUIDS, 

Dejinition of a Fluid ; Distinction between Fluids and 
Liquids ; Of Non-Elastic Fluids, Scarcely Susceptible 
of Compression ; Of the Cohesion of Fluids ; Of their 
Gravitation ; Of their Equilibrium ; Of their Pressure ; 
Of Specific Gravity ; Of the Specific Gravity oj Bodies 
Heavier than Water ; Of those of the Same Weight as 
Water ; Of those Lighter than Water ; Of the Specific 
Gravity of Fluids* 



456. What is the science called, that treats of the mechan- 
ical properties of fluids ? 

457. What is a fluid ? 

458. In which is the attraction of cohesion the most pow- 
erful, solids or fluids ? 

459. What is the distinction between a liquid and a fluid .^ 

460. Are water and other liquids compressible ? 

461. Why are they not ? 

462. What reason is therefor supposing that the parti- 
cles of fluids are round ? 

463. What experiment has been made to shew that liquids 
are not compressible ^^ 

464. Why cannot liquids be moulded into figures, like 
solids ? 

465. What is meant by the level or equihbrium of fluids ? 

466. Why will oil remain upon the top of water ? 

467. Why is the resistance of fluids less than that of solids ? 

468. Why are fluids inclined to a state of equilibrium ? 

469. Why will water and other liquids run out of any 
opening to the vessel containing them ? 

470. From what does the lateral pressure of liquids pro- k 
ceed ? I 

471. To what is the velocity proportioned of liquids issu- 
ing from an orifice in the side of a vessel containing them ? 



OP SPRINGS, FOUNTAINS; &C. 235 

472. How would you explain figure 5, Plate XIII , ? 

473. From what proceeds the upward pressure of liquids ? 

474. How would you explain figure 4, Plate XIII. ? 

475. What is the object of figure 6, Plate XIII. ? 

476. What is the specific gravity of a body ? 

477. To what is the specific gravity of bodies propor- 
tioned ? 

478. What standard has been fixed on to determine the 
specific gravity of different bodies ? 

479. Why will a body weigh less in the water than out 
of it? 

480. To what is the resistance of water to a body im- 
mersed in it proportioned ? 

481. How much does a body weighed in the water lose of 
its weight ? 

482. What is the specific gravity of gold ? 

483. What solid is there of the same specific gravity of 
water ? 

484. How will a body of the specific gravity of water re- 
main in water ? 

485. How is the specific gravity of fluids ascertained ? 

486. How would you explain figure 8, Plate XIII. ? 



QUESTIONS TO CONVERSATION XI. 



OF SPRINGS, FOUNTAINS, &c. 

Of the Ascent of Vapor and the Formation of Clouds ; Of 
the Formation and Fall of Rain^ ^c. ; Of the Forma-" 
Hon of Springs ; Of Rivers and Lakes ; Of Fountains. 



487. What is the reason that the great quantity of rain 
which falls upon the earth and sinks into it, does not, in the 
course of time, injure its solidity ? 

488. How are clouds formed ? 

489. But since the watery vapor is lighter than the air, 
why does it not continue to rise ; and why does it unite again 
to form clouds ? 



240 MECHANICAL PROPERTIJES OP AIR. 

490. What prevents the clouds remaining in the atmos^ 
phere, where they are formed ? 

491. Why do the clouds descend to the earth in drops of 
water instead of vapor, as they ascend ? 

492. What is the difference between rain and spring 
water ? 

493. Which is the most pure ? 

494. How are rivulets, at first, formed ? 

495. How would you explain figure 9> Plate XIII. ? 

496. How high may a spring rise ? 

497. On what principle does water ascend as well as de- 
scend in its course, as is often the case, in carrying it by the 
use of ducts ? 

498. How would you explain figure 1, Plate XIV. ? 

499. What is the name of the cup made on the principle 
of this figure ? 

500. Why must wells on high land, as on hills, be dug 
deep in order to be supplied with water r 

501. Why do rivers generally have their source in moun- 
tainous regions ? 

502. How was the lake Geneva probably formed ? 

503. How would you explain figure 2, Plate XIV. ? 



QUESTIONS TO CONVERSATION XH. 



ON THE MECHANICAL PROPERTIES OF AIR. 

Of the Spring or Elasticity of the Air ; Of the weight of 
the Air ; Experiments ivith the Air Pump ; Of the Ba- » 
rometer ; Mode of Weighing Air ; Specific Gravity of\ 
Air ; Of Pumps ; Description of the Sucking Pump ; 
Description of the Forcing Pump. 



504. How are the fluids called air distinguished from 
liquids ? 

505. What effect does heat have on elastic fluids ? , 

506. To what distance from the earth does the atmosphere^) 
extend ? 



MECHANICAL PROPERTIES OF AIR. 241 

507. What weight of air is a common or middling size 
man supposed to sustain ? 

508. Why does not such a weight crush him to atoms ? 

509. What would be the consequence if the weight of 
external air were removed from us ? 

510. Why do not bodies of various weights in the open 
air fall in the same time ? 

511. How may it be shown that the air has weight ? 

512. How may the power of expansion in air be ascer* 
tained ? 

513. How much does a column of air reaching to the top 
of the atmosphere of an inch in diameter, weigh ? 

514. How can the weight of a small quantity of air be 
ascertained ? 

515. How much heavier Is water than air ? 

516. How can the weight of the atmosphere be determined 
by a Barometer ? 

517. When is the air the heaviest, in wet or in dry weather? 

518. But why do our feelings indicate that the air is 
heaviest in wet weather, if that is not the fact ? 

519. Is the atmosphere of the same density on a high hill 
or mountain as in a valley ? 

520. How may the height of mountains be ascertained by 
R Barometer ? 

521. Does a person in such elevated situations feel any 
inconvenience from the thinness of the atmosphere ? 

522. On what principle is the thermometer constructed ? 

523. When are two fluids of different density in equili- 
brium ? 

524. What causes water to rise in a pump ? 

525. How high will it rise ? 

526. What is the construction of a pump ? 

527. How would you explain figure 4, Plate XIV. ? 

528. On what principle is a liquid sucked through a straw 
or any small tube ? 

529. How would you explain figure 5, Plate XIV. ? 



21 



QUESTIONS TO CONVERSATION XIII. 



ON WIND AND SOUND. 



Of Wind in General ; Of the Trade Wind; Of the 
Periodical Trade Winds ; Of the Aerial Tides ; Of 
Sounds in General ; Of Sonorous Bodies ; Of Musical 
Sounds ; Of Concord or Harmony ^ and Melody, 



530. What is wind ? 

531. How is the air put in motion so as to produce wind ? 

532. What is the consequence where the winds from 
different quarters meet or interfere ? 

533. Where does this mostly happen ? 

534. What regular wind prevails at the equator ? 

535. Why is there a regular east wind at and near the 
equator ? 

^S6, How are the trade winds^ as they are called^ pro- 
duced ? 

bSJ, Why do not the polar regions become exhausted of 
air, if it is continually blowing from them to the equator ? 

538. What familiar illustration or example can you give of 
the circulation of air — first from the poles to the equator^ and 
then rising and returning to the poles ? 

539. Why are the periodical winds more regular at sea 
than on land ? 

540. What winds are called monsoons ? 

541. Why is it that the wind north of the equator does not 
regularly blow south ; and that on the south of the equator 
regularly north, according to the above hypothesis, instead of 
being almost continually variable as they now are ? 

542. What are the sea breezes, as they are termed ? 

543. Why does the wind generally subside at the going 
down of the sun ? 

544. Does the moon have any effect on the wind ? 

545. Is there any difference of weight to a column of 
atmosphere, when under the influence of the moon's attrac 
tion. from other times ? 



ON \V1ND AND SOUND. 243 

546. Why is there not ? 

547. AVhat produces sound ? 

548. How can it be shewn that air is necessary to the pro- 
duction of sound ? 

549. Is the atmosphere the only conductor of sound ? 

550. How can it be shewn that solids are conductors of 
sound ? 

551. What bociies are called sonorous ? 

552. To what do they owe their sonorous property ? 

553. How would you explain figure 6. Plate XIV. ? 

554. To what is the tremulous motion given to the air by a 
sonorous body compared ? 

555. W^hy is motion more easily communicated to air than 
to water ? 

556. AVhy do we see the flash of a cannon at a distance, 
before we hear the report ? 

5d7' What is the velocity of sound ? 

558. How is the sound of an echo produced ? 

559. How is harmony or concord in sounds produced ? 

560. How is an octave concord produced ? 

561. How is that species of harmony called a fifth pro- 
duced ? 



QUESTIONS TO CONVERSATION XIV. 



OPTICS. 

Of LinninouSy Transparent^ and Opaque Bodies ; Of the 
Radiation of Light ; Of Shadoivs ; Of the Reflection 
of Light ; Opaque Bodies seen only hy Reflected Light ; 
Vision Explained ; Camera Ohscura ; Image of Objects 
on the Retina, 



562. What is the science called that treats of vision 

563. What is a luminous body ? 

564. What is an opaque body ? 
^6^\ What are transparent bodies ? 



244 OPTICS. 

566* What are transparent bodies frequently called, wheu 
spoken of philosophically ? 

567' In what manner is light projected from luminous 
bodies ? 

568. Do the rays of light which cross each other impede * 
each other's course ? 

569. Why do they not ? 

570. What is a ray of light ? 

571. What is a pencil of light ? 

572. Is light a substance composed of particles like other 
bodies ? 

573. In what respect is liglit subject to the laws of matter ? 

574. In what respect is it not subject to the laws of matter ? 

575. What is the consequence when rays of light fall upon 
an opaque body ? 

576. What produces darkness ? 
577' What produces a shadow ? 

578. Why are shadows of different degrees of darkness ? 

^79' When a shadow^ is produced by the interruption of 
rays of light from a single luminous body, to what is the 
darkness of the shadow proportioned ? 

580. Why does a total eclipse of the sun occasion a mor€ 
sensible darkness than midnight ? 

581. What will be the form of a shadow when the lumin- 
ous body is larger thra the opaque body upon which it shines ? 

582. >Vhat will be the form of the shadow when the 
opaqup Dod}^ is the largest ? 

583. Why is it, that shadows produced by the intervention 
of terrestrial objects in the sun and moon's light, are of increas- 
ed size, instead of terminating in a point according to the 
general principle ? 

584. How may more shadows than one be produced by a 
single opaque body ? 

5Sd. What is the reflection of light ? 

586. Is all the light that falls upon an opaque body 
reflected ? 

587. How will a ray of light be reflected that falls upon an 
opaque body perpendicularly ? 

588. How will one be reflected that falls upon an opaque 
body obliquely ? 

589. What is the angle of incidence ? 

590. What is an angle of reflection ? 

501. By what rays do we see opaque bodies ? 



OPTICS. 245 

592. How are we able to see light that falls upon an 
opaque body and is reflected, but not in a direction to meet 
the eye ? 

593. Why does one side of an opaque body appear to be 
in the sunshine and the other side in the shade ^ when by not 
seeing the rays that fall upon the object, both sides of it 
would appear shaded ? 

594. Why is it that the whole of a surface of water on 
which the sun shines does not appear illumined ? 

595. Why is it that objects on a hill appear more distinct 
than at an equal distance from us in a valley ? 

596. How is it that the rays of light shew us or give us an 
idea of the objects from whiclithey proceed ? 

597. What is camera obscura ? 

598. How does a camera obscura represent the manner in 
which objects are pictured upon the retina of the eye ? 

599. Why is it that the objects exhibited by a camera 
obscura are inverted ? 

600. When an object is pictured upon the retina of the 
eye, how is the idea of the object conveyed to the mind ? 

601. If objects are seen only by their pictures on the retina 
of the eye, why do they not appear inverted, as in the camera 
obscura ? 

602. What is the reason that objects appear smaller at a 
distance than they really are ? 

603. What is the a»gle of vision ? 

604. What is the size of the angle of vision proportioned 
to? 

605. Why is it that objects of the same size with which 
we are acquainted are known to be of the same size, if they 
form pictures of unequal sizes upon the retina of the eye ? 

606. Why is it, that objects viewed in front appear larger 
than when viewed obliquely ? 

607. On what principle are the laws of perspective 
founded ? 

608. How is nature to be exhibited in sculpture ? 

609. How in painting ? 

610. When are objects invisible ? 

611. Why is it that the motion of the celestial bodies is 
invisible ? 

612. How would objects appear, as to distance, to a per- 
son, who had always been blind, on first being made to see ? 

613. Why would they seem t© touch the eye ? 

21* 



,.'40 or TIC.-. 

614. If the image of an object is formed on the retina oi 
each eye, why does not the object appear double ? 

61 5. When we see the image of an object in a looking 
glasSj why does it not appear inverted^ as in the camera 
obscura ? 

616. Explain figure 3, Plate XVII. 

617. Why is the ray C D reflected just so as to meet the 
eye at A. ? 

618. Why may we not see ourselves entire in a looking 
glass, less than half our own length ? 

61 9- W^hy cannot persons see their own image in a looking 
glass, when they stand obliquely to the right or left of the 
glass ? 

620. If you stand obliquely to the right of the glass, why 
is it that another person must stand just as much to the left of 
the glass in order to see your image ? 

621. W hen you stand at the right of the glass and I stand 
at the left of it, why does your image appear directly oppo- 
site yourself ? 

G22. If all opaque bodies reflect light, why is it, that we 
cannot as well see ourselves when looking at any other ob- 
ject, as when viewing a looking glass ? 

623. How many kinds of mirrors are there used in optics ? 

624. What are they ? 

625. How does a convex mirror exhibit an object ? 

626. How does a concave mirror ? 

627. Vv hat is a focus in optics ? 

628. What is an imaginary focus ? 

629. Explain figure 1, Plate XVIII. 

630. Explain figure 2, Plate XVIII. 

631. What is the focus of a concave mirror ? 

632. Will the focus be in the same place whether the rays 
fall parallel or converging upon the mirror ? 

GSS, Which focus is most distant from the mirror ? 
634. Will the focus be in the same place whether the rays 
fall parallel or diverging upon the mirror ? 

Go J. Which will be the farthest from the mirror ? 
62>6, W^hat are concave mirrors sometimes called ? 

637. W^hy are they called burning glasses ? 

638. Why does a convex mirror make objects appeal 
smaller than they are ? 

639. If a light is placed in the focus of a concave mirror, 
how will the rays fall upon the mirror ? 



ON REFHACTICN AND COLGflS. 247 

640. Where must the object be placed in regard to a con- 
cave mirror in order that it may be magnified ? 

641. Why is the image in a concave mirror larger than 
the object ? 



QUESTIONS TO CONVERSATION XVL 



ON REFRACTION AND COLORS. 

Transmission of Light hy Transparent Bodies ; Refrac- 
tion ; Refraction of the Atmosphere ; Refraction of a 
Lens ; Refraction of the Prism ; Of the Colors of Raji^ 
of Light ; Of the Colors of Bodies. 



642. What is the refraction of light ? 

643. When does refraction in light take place ? 

644. What causes refraction ? 

645. Explain the figure 1^ Plate XIX. 

646. Why does the ray C. B. descend to F. instead of D. 
or E. ? 

647. Why does a straight stick appear crooked when one 
end of it is placed obliquely in the water ? 

648. Explain figure 2, Plate XIX. 

649. Why is it, that water in a river or brook or in any 
vessel appears more shallow than it really is ? 

650. In what situation may we view the bottom of a water, 
so that it will appear of its real depth ? 

651. Do the sun and the other heavenly bodies appear to 
us in their real situation ? 

652. Explain figure 4, Plate XIX. 

6ij3, Why do not the sun and the other heavenly bodies 
appear in their real situation ? 

654. In what situation may the sun be seen in its true 
place ? 

655. Wliat besides the refraction of light is there to pre- 
vent our seeing the sun in its real situation ? 

656. How long is light in comhig from the sun to the earth ? 



^7 18 ON REFRACTION AND COLORS, 

Gd7* How is it that our days appear longer than they 
really are ? 

658. Is light refracted in passing through a common win- 
dow glass ? 

659. How would you explain figure 5, Plate XIX. ? 

660. Do objects seen through common window glass ap- 
pear in a different place from that in which they really are ? 

661. Why do they not, since they are seen by rays of light 
that are refracted ? 

662. What is a lens ? 

663. In parallel rays that fall upon a convex lens, what 
ones will be refracted ? 

664. To what will they be refracted ? 

665. What is the axis of a lens ? 

666. How would you explain figure 6, Plate XIX. 9 
667' What is the distance of the focus from the surface of 

the lens ? 

668. What is the object or property of a convex lens ? 

669. W^hat is the object or property of a concave lens ? 

670. What is the object of figure 7, Plate XIX. ? 

671. What is a radius ? 

672. W^hat is a plano-convex lens ? 

673. AVhat is a plano-concave lens.^ 

674. How would you explain figure 2, Plate XX. ? 
675' What is a prism ? 

676. How would you explain figure 3, Plate XX. ? 
677' Of what do the rays of the sun consist ? 

678. How many colors are there in a ray of light ? 

679. How can these colors be separated ? 

680. What are the colors of a ray of light ? 

681. How would you explain figure 4, Plate XX. ? 

682. Why do the colors of a ray of light separate in pas- 
sing throur^h a prism ? 

683. Hew can these colors once separated be again united ? 

684. What is the object of figure 5, Plate XX. ? 

685. What composes white ? 

686. What simple ilhistration can be given of these seven 
colors makirg white on beirg united ? 

687. What causes the rainbow ? 

688. When is a lens called a burning glass ? 

689. Why will a ^'iece of brown paper placed beneath a 
lens which collects the sun^s rays take fire sooner than a piece 
of white paper ? 



OPTICS. 249 

690. What colors do different objects reflect and what 
ones do they absorb ? 

691. How do we know, that opaque bodies absorb all the 
colors, that are not their own, and that they reflect their own 
and none others ? 

692. Are colors essential properties of bodies ? 

693. On what do they depend ? 

694. To what is darkness of color owing ? 

695. Why does blue often appear of a greenish cast by 
candle hght ? 

696. Why are people of a sallow or yellow complexion 
fairer in the night, if the candle light gives all bodies a yel- 
lowish tinge ? 

697' W^hy does the sun appear red through the clouds ? 

698. Why does It appear red in the morning ? 

699. Why does the sky appear blue ? 

700. Why is a black di'ess warmer than a white one ? 

701. Why is a white one more dazzling than a black one ? 



QUESTIONS TO CONVERSATION XVIL 



ON THE STRUCTURF^ OF THE EYE AND OPTICAL 
INSTRUMENTS. 

Description of the Eye ; Of the Image on the Retina ; 
Refraction of the Humors of the Eye ; Of the Use of 
Spectacles ; Of the Single Microscope ; Of the Double 
Microscope ; Of the Solar Microscope ; Magic Lan- 
thorn; Refracting Telescope ; Refecting Telescope, 



702. What is the form of the body of the eye ? 

703. W^hat is the sclerotica of the eye ? 

704. What is the cornea of the eye ? 

705. What is the choroid of the eye ? 

706. What is the pupil of the eye ? 

707. What is the iris ? 

708. Is the pupil always of the same size ? 



,30 OPTICS. 

709. When does it enlarge ? 

710. When is it contracted ? 

711' Why does it give the eyes pain on first going into a 
bright light from a dark room ? 

719., Why does it seem much darker on first going out in 
the night than after we have been out a short time ? 

713. How much more Hght is admitted when the pupil is 
extended to the utmost, than when most contracted ? 

714. Why can cats, horses, and some other animals, see so 
much better in the night than we can ? 

71 ^^ How much is it thought that the pupil of their eyes 
extend and contract ? 

716. What becomes of the rays that fall on the choroid? 

717. W^hat are the transparent substances called enclosed 
within the membraneous coverings ? 

718. How many humors are there ? 

719. What are they called ? 

720. From what does the aqueous humor derive its name ? 

721. From what does the chrystalline humor derive its 
name ? 

722. From what does the vitreous humor derive its 
name? 

723. For what are the membraneous coverings of the eye 
chiefly intended ? 

724. Explain figure 1, Plate XXI. 

725. What is the us« of the refracting humors ? 

726. Explain figure 3, Plate XXI. 

727. Explain figure 4, Plate XXI. 

728. How does the refracting humor remedy the defects 
exhibited in figure 4 ? 

729. Why is not something like the refracting humors 
necessary in the camera obscura ? 

730. What causes some persons to be short-sighted, as it 
is termed ? 

731. Why can short-sighted persons see better by bringing 
the objects near to the eyes ? 

732. What remedy have short-sighted persons in viewing 
distant objects ? 

733. Explain figure 1, Plate XXII. 

734. What is the reason that old people lose their sight ? 

735. Explain figure 2, Plate XXII. 

736. Why do old persons without convex glasses hold the 
objects to be seen at a distance from the eye ? 



QPTICS. 251 

737. Why cannot we see distinctly when the object is 
placed close to the eye ? 

738. Explain figure 4, Plate XXII. 

739. In what way can objects be seen distinctly when 
placed near the eye ? 

740. What is a microscope ? 

741. Explain figure 5^ Plate XXII. 

742. How is the magnifying power in the microscope 
increased ? 

743. What is a double microscope? 

744. How must an object be placed in regard to a lenS;, 
so that the object be magnified ? 

745. How so that the object be diminished .^ 

746. How would you explain Figure 1, Plate XXIII. ? 

747. How would you explain Figure 2, Plate XXIII. ? 

748. What is the design of Figure 3, Plate XXIII. ? 

749. How does a magic lanthorn differ from the double 
lens and mirror ? 

750. What is a magic lanthorn ? 

751. What is the design of Figure 4, Plate XXIII. ? 

752. How would you explain it ? 



DIRECTIONS 

FOR PLACING THE ENGRAVINGS. 

Plate I. to face page 31 

II 49 

III 65 

IV 61 

V 68 

VI 78 

VII 89 

VIII 93 

IX 100 

X 110 

XI 114 

XII 118 

XIII 128 

XIV 141 

XV 167 

XVI 173 

XVII 178 

XVIIl 186 

XIX 190 

XX 195 

XXI 205 

XXII 209 

XXIII 212